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Gurusamy Sarathy


perldelta - what's new for perl v5.6 (as of v5.005_64)


This is an unsupported alpha release, meant for intrepid Perl developers only. The included sources may not even build correctly on some platforms. Subscribing to perl5-porters is the best way to monitor and contribute to the progress of development releases (see http://www.hut.fi/~jhi/perl5-porters.html for info).

This document describes differences between the 5.005 release and this one.

Incompatible Changes

Perl Source Incompatibilities

Beware that any new warnings that have been added or old ones that have been enhanced are not considered incompatible changes.

Since all new warnings must be explicitly requested via the -w switch or the warnings pragma, it is ultimately the programmer's responsibility to ensure that warnings are enabled judiciously.

CHECK is a new keyword

In addition to BEGIN, INIT, END, DESTROY and AUTOLOAD, subroutines named CHECK are now special. These are queued up during compilation and behave similar to END blocks, except they are called at the end of compilation rather than at the end of execution. They cannot be called directly.

Treatment of list slices of undef has changed

When taking a slice of a literal list (as opposed to a slice of an array or hash), Perl used to return an empty list if the result happened to be composed of all undef values.

The new behavior is to produce an empty list if (and only if) the original list was empty. Consider the following example:

    @a = (1,undef,undef,2)[2,1,2];

The old behavior would have resulted in @a having no elements. The new behavior ensures it has three undefined elements.

Note in particular that the behavior of slices of the following cases remains unchanged:

    @a = ()[1,2];
    @a = (getpwent)[7,0];
    @a = (anything_returning_empty_list())[2,1,2];
    @a = @b[2,1,2];
    @a = @c{'a','b','c'};

See perldata.

Perl's version numbering has changed

Beginning with Perl version 5.6, the version number convention has been changed to a "dotted integer" scheme that is more commonly found in open source projects.

Maintenance versions of v5.6.0 will be released as v5.6.1, v5.6.2 etc. The next development series following v5.6 will be numbered v5.7.x, beginning with v5.7.0, and the next major production release following v5.6 will be v5.8.

The English module now sets $PERL_VERSION to $^V (a string value) rather than $] (a numeric value). (This is a potential incompatibility. Send us a report via perlbug if you are affected by this.)

The v1.2.3 syntax is also now legal in Perl. See "Support for strings represented as a vector of ordinals" for more on that.

To cope with the new versioning system's use of at least three significant digits for each version component, the method used for incrementing the subversion number has also changed slightly. We assume that versions older than v5.6 have been incrementing the subversion component in multiples of 10. Versions after v5.6.0 will increment them by 1. Thus, using the new notation, 5.005_03 is the same as v5.5.30, and the first maintenance version following v5.6.0 will be v5.6.1, which amounts to a floating point value of 5.006_001).

Literals of the form 1.2.3 parse differently

Previously, numeric literals with more than one dot in them were interpreted as a floating point number concatenated with one or more numbers. Such "numbers" are now parsed as strings composed of the specified ordinals.

For example, print 97.98.99 used to output 97.9899 in earlier versions, but now prints abc.

See "Support for strings represented as a vector of ordinals" below.

Possibly changed pseudo-random number generator

In 5.005_0x and earlier, perl's rand() function used the C library rand(3) function. As of 5.005_52, Configure tests for drand48(), random(), and rand() (in that order) and picks the first one it finds. Perl programs that depend on reproducing a specific set of pseudo-random numbers will now likely produce different output. You can use sh Configure -Drandfunc=rand to obtain the old behavior.

Hashing function for hash keys has changed

Perl hashes are not order preserving. The apparently random order encountered when iterating on the contents of a hash is determined by the hashing algorithm used. To improve the distribution of lower bits in the hashed value, the algorithm has changed slightly as of 5.005_52. When iterating over hashes, this may yield a random order that is different from that of previous versions.

undef fails on read only values

Using the undef operator on a readonly value (such as $1) has the same effect as assigning undef to the readonly value--it throws an exception.

Close-on-exec bit may be set on pipe and socket handles

On systems that support a close-on-exec flag on filehandles, the flag will be set for any handles created by pipe(), socketpair(), socket(), and accept(), if that is warranted by the value of $^F that may be in effect. Earlier versions neglected to set the flag for handles created with these operators. See "pipe" in perlfunc, "socketpair" in perlfunc, "socket" in perlfunc, "accept" in perlfunc, and "$^F" in perlvar.

Writing "$$1" to mean "${$}1" is unsupported

Perl 5.004 deprecated the interpretation of $$1 and similar within interpolated strings to mean $$ . "1", but still allowed it.

In Perl 5.6 and later, "$$1" always means "${$1}".

delete(), values() and \(%h) operate on aliases to values, not copies

delete(), each(), values() and hashes in a list context return the actual values in the hash, instead of copies (as they used to in earlier versions). Typical idioms for using these constructs copy the returned values, but this can make a significant difference when creating references to the returned values.

Keys in the hash are still returned as copies when iterating on a hash.

vec(EXPR,OFFSET,BITS) enforces powers-of-two BITS

vec() generates a run-time error if the BITS argument is not a valid power-of-two integer.

Text of some diagnostic output has changed

Most references to internal Perl operations in diagnostics have been changed to be more descriptive. This may be an issue for programs that may incorrectly rely on the exact text of diagnostics for proper functioning.

%@ has been removed

The undocumented special variable %@ that used to accumulate "background" errors (such as those that happen in DESTROY()) has been removed, because it could potentially result in memory leaks.

Parenthesized not() behaves like a list operator

The not operator now falls under the "if it looks like a function, it behaves like a function" rule.

As a result, the parenthesized form can be used with grep and map. The following construct used to be a syntax error before, but it works as expected now:

    grep not($_), @things;

On the other hand, using not with a literal list slice may not work. The following previously allowed construct:

    print not (1,2,3)[0];

needs to be written with additional parentheses now:

    print not((1,2,3)[0]);

The behavior remains unaffected when not is not followed by parentheses.

Semantics of bareword prototype (*) have changed

Arguments prototyped as * will now be visible within the subroutine as either a simple scalar or as a reference to a typeglob. Perl 5.005 always coerced simple scalar arguments to a typeglob, which wasn't useful in situations where the subroutine must distinguish between a simple scalar and a typeglob. See "Prototypes" in perlsub.

On 64-bit platforms the semantics of bit operators have changed

If your platform is either natively 64-bit or your Perl has been configured to used 64-bit integers (say perl -V and see what is your ivsize: if it is 8, you are 64-bit) , be warned that the semantics of all the bitwise numeric operators (& | ^ ~ << >>) have been changed. They used to be forced to be 32 bits wide, but now in the aforementioned platforms they are 64 bits wide. Most dramatically this affects the unary ~: what used to be 32 bits wide, is now 64 bits wide. If you depend on your integers being 32 bits wide, mask off the excess bits with & 0xffffffff.

C Source Incompatibilities


Release 5.005 grandfathered old global symbol names by providing preprocessor macros for extension source compatibility. As of release 5.6, these preprocessor definitions are not available by default. You need to explicitly compile perl with -DPERL_POLLUTE to get these definitions. For extensions still using the old symbols, this option can be specified via MakeMaker:

    perl Makefile.PL POLLUTE=1

PERL_IMPLICIT_CONTEXT is automatically enabled whenever Perl is built with one of -Dusethreads, -Dusemultiplicity, or both. It is not intended to be enabled by users at this time.

This new build option provides a set of macros for all API functions such that an implicit interpreter/thread context argument is passed to every API function. As a result of this, something like sv_setsv(foo,bar) amounts to a macro invocation that actually translates to something like Perl_sv_setsv(my_perl,foo,bar). While this is generally expected to not have any significant source compatibility issues, the difference between a macro and a real function call will need to be considered.

This means that there is a source compatibility issue as a result of this if your extensions attempt to use pointers to any of the Perl API functions.

Note that the above issue is not relevant to the default build of Perl, whose interfaces continue to match those of prior versions (but subject to the other options described here).

See "The Perl API" in perlguts for detailed information on the ramifications of building Perl using this option.


Enabling Perl's malloc in release 5.005 and earlier caused the namespace of system versions of the malloc family of functions to be usurped by the Perl versions, since by default they used the same names.

Besides causing problems on platforms that do not allow these functions to be cleanly replaced, this also meant that the system versions could not be called in programs that used Perl's malloc. Previous versions of Perl have allowed this behaviour to be suppressed with the HIDEMYMALLOC and EMBEDMYMALLOC preprocessor definitions.

As of release 5.6, Perl's malloc family of functions have default names distinct from the system versions. You need to explicitly compile perl with -DPERL_POLLUTE_MALLOC to get the older behaviour. HIDEMYMALLOC and EMBEDMYMALLOC have no effect, since the behaviour they enabled is now the default.

Note that these functions do not constitute Perl's memory allocation API. See "Memory Allocation" in perlguts for further information about that.

Compatible C Source API Changes


The cpp macros PERL_REVISION, PERL_VERSION, and PERL_SUBVERSION are now available by default from perl.h, and reflect the base revision, patchlevel, and subversion respectively. PERL_REVISION had no prior equivalent, while PERL_VERSION and PERL_SUBVERSION were previously available as PATCHLEVEL and SUBVERSION.

The new names cause less pollution of the cpp namespace and reflect what the numbers have come to stand for in common practice. For compatibility, the old names are still supported when patchlevel.h is explicitly included (as required before), so there is no source incompatibility from the change.

Support for C++ exceptions

change#3386, also needs perlguts documentation [TODO - Chip Salzenberg <chip@perlsupport.com>]

Binary Incompatibilities

In general, the default build of this release is expected to be binary compatible for extensions built with the 5.005 release or its maintenance versions. However, specific platforms may have broken binary compatibility due to changes in the defaults used in hints files. Therefore, please be sure to always check the platform-specific README files for any notes to the contrary.

The usethreads or usemultiplicity builds are not binary compatible with the corresponding builds in 5.005.

On platforms that require an explicit list of exports (AIX, OS/2 and Windows, among others), purely internal symbols such as parser functions and the run time opcodes are not exported by default. Perl 5.005 used to export all functions irrespective of whether they were considered part of the public API or not.

For the full list of public API functions, see perlapi.

Installation and Configuration Improvements

-Dusethreads means something different

WARNING: Support for threads continues to be an experimental feature. Interfaces and implementation are subject to sudden and drastic changes.

The -Dusethreads flag now enables the experimental interpreter-based thread support by default. To get the flavor of experimental threads that was in 5.005 instead, you need to run Configure with "-Dusethreads -Duse5005threads".

As of v5.5.640, interpreter-threads support is still lacking a way to create new threads from Perl (i.e., use Thread; will not work with interpreter threads). use Thread; continues to be available when you ask for use5005threads, bugs and all.

New Configure flags

The following new flags may be enabled on the Configure command line by running Configure with -Dflag.

    usethreads useithreads      (new interpreter threads: no Perl API yet)
    usethreads use5005threads   (threads as they were in 5.005)

    use64bitint                 (equal to now deprecated 'use64bits')

    usesocks                    (only SOCKS v5 supported)

Threadedness and 64-bitness now more daring

The Configure options enabling the use of threads and the use of 64-bitness are now more daring in the sense that they no more have an explicit list of operating systems of known threads/64-bit capabilities. In other words: if your operating system has the necessary APIs and datatypes, you should be able just to go ahead and use them, for threads by Configure -Dusethreads, and for 64 bits either explicitly by Configure -Duse64bitint or implicitly if your system has 64 bit wide datatypes. See also "64-bit support".

Long Doubles

Some platforms have "long doubles", floating point numbers of even larger range than ordinary "doubles". To enable using long doubles for Perl's scalars, use -Duselongdouble.


You can enable both -Duse64bitint and -Dlongdouble by -Dusemorebits. See also "64-bit support".


Some platforms support large files, files larger than two gigabytes. See "Large file support" for more information.


You can use "Configure -Uinstallusrbinperl" which causes installperl to skip installing perl also as /usr/bin/perl. This is useful if you prefer not to modify /usr/bin for some reason or another but harmful because many scripts assume to find Perl in /usr/bin/perl.

SOCKS support

You can use "Configure -Dusesocks" which causes Perl to probe for the SOCKS (v5, not v4) proxy protocol library, http://www.socks.nec.com/

-A flag

You can "post-edit" the Configure variables using the Configure -A flag. The editing happens immediately after the platform specific hints files have been processed but before the actual configuration process starts. Run Configure -h to find out the full -A syntax.

Enhanced Installation Directories

The installation structure has been enriched to improve the support for maintaining multiple versions of perl, to provide locations for vendor-supplied modules, scripts, and manpages, and to ease maintenance of locally-added modules, scripts, and manpages. See the section on Installation Directories in the INSTALL file for complete details. For most users building and installing from source, the defaults should be fine.

If you previously used Configure -Dsitelib or -Dsitearch to set special values for library directories, you might wish to consider using the new -Dsiteprefix setting instead. Also, if you wish to re-use a config.sh file from an earlier version of perl, you should be sure to check that Configure makes sensible choices for the new directories. See INSTALL for complete details.

Core Changes

Unicode and UTF-8 support

Perl can optionally use UTF-8 as its internal representation for character strings. The utf8 and bytes pragmas are used to control this support in the current lexical scope. See perlunicode, utf8 and bytes for more information.

Interpreter cloning, threads, and concurrency

WARNING: This is an experimental feature in a pre-alpha state. Use at your own risk.

Perl 5.005_63 introduces the beginnings of support for running multiple interpreters concurrently in different threads. In conjunction with the perl_clone() API call, which can be used to selectively duplicate the state of any given interpreter, it is possible to compile a piece of code once in an interpreter, clone that interpreter one or more times, and run all the resulting interpreters in distinct threads.

On Windows, this feature is used to emulate fork() at the interpreter level. See perlfork.

This feature is still in evolution. It is eventually meant to be used to selectively clone a subroutine and data reachable from that subroutine in a separate interpreter and run the cloned subroutine in a separate thread. Since there is no shared data between the interpreters, little or no locking will be needed (unless parts of the symbol table are explicitly shared). This is obviously intended to be an easy-to-use replacement for the existing threads support.

Support for cloning interpreters and interpreter concurrency can be enabled using the -Dusethreads Configure option (see win32/Makefile for how to enable it on Windows.) The resulting perl executable will be functionally identical to one that was built with -Dmultiplicity, but the perl_clone() API call will only be available in the former.

-Dusethreads enables, the cpp macros USE_ITHREADS by default, which enables Perl source code changes that provide a clear separation between the op tree and the data it operates with. The former is considered immutable, and can therefore be shared between an interpreter and all of its clones, while the latter is considered local to each interpreter, and is therefore copied for each clone.

Note that building Perl with the -Dusemultiplicity Configure option is adequate if you wish to run multiple independent interpreters concurrently in different threads. -Dusethreads only provides the additional functionality of the perl_clone() API call and other support for running cloned interpreters concurrently.

[XXX TODO - the Compiler backends may be broken when USE_ITHREADS is enabled.]

Lexically scoped warning categories

You can now control the granularity of warnings emitted by perl at a finer level using the use warnings pragma. See warnings and perllexwarn for details.

Lvalue subroutines

WARNING: This is an experimental feature.

change#4081 [TODO - Ilya Zakharevich <ilya@math.ohio-state.edu>, Tuomas Lukka <lukka@iki.fi>)]

"our" declarations

An "our" declaration introduces a value that can be best understood as a lexically scoped symbolic alias to a global variable in the package that was current where the variable was declared. This is mostly useful as an alternative to the vars pragma, but also provides the opportunity to introduce typing and other attributes for such variables. See "our" in perlfunc.

Support for strings represented as a vector of ordinals

Literals of the form v1.2.3.4 are now parsed as a string composed of of characters with the specified ordinals. This is an alternative, more readable way to construct (possibly unicode) strings instead of interpolating characters, as in "\x{1}\x{2}\x{3}\x{4}". The leading v may be omitted if there are more than two ordinals, so 1.2.3 is parsed the same as v1.2.3.

Strings written in this form are also useful to represent version "numbers". It is easy to compare such version "numbers" (which are really just plain strings) using any of the usual string comparison operators eq, ne, lt, gt, etc., or perform bitwise string operations on them using |, &, etc.

In conjunction with the new $^V magic variable (which contains the perl version as a string), such literals can be used as a readable way to check if you're running a particular version of Perl:

    # this will parse in older versions of Perl also
    if ($^V and $^V gt v5.5.640) {
        # new features supported

require and use also have some special magic to support such literals. They will be interpreted as a version rather than as a module name:

    require v5.6.0;             # croak if $^V lt v5.6.0
    use v5.6.0;                 # same, but croaks at compile-time

Alternatively, the v may be omitted if there is more than one dot:

    require 5.6.0;
    use 5.6.0;

Also, sprintf and printf support the Perl-specific format flag %v to print ordinals of characters in arbitrary strings:

    printf "v%vd", $^V;         # prints current version, such as "v5.5.650"
    printf "%*vX", ":", $addr;  # formats IPv6 address
    printf "%*vb", " ", $bits;  # displays bitstring

See "Strings of Character" in perlop for additional information.

Weak references

WARNING: This is an experimental feature.

In previous versions of Perl, you couldn't cache objects so as to allow them to be deleted if the last reference from outside the cache is deleted. The reference in the cache would hold a reference count on the object and the objects would never be destroyed.

Another familiar problem is with circular references. When an object references itself, its reference count would never go down to zero, and it would not get destroyed until the program is about to exit.

Weak references solve this by allowing you to "weaken" any reference, that is, make it not count towards the reference count. When the last non-weak reference to an object is deleted, the object is destroyed and all the weak references to the object are automatically undef-ed.

To use this feature, you need the WeakRef package from CPAN, which contains additional documentation.

change#3385, also need perlguts documentation [TODO - Tuomas Lukka <lukka@iki.fi>]

File globbing implemented internally

WARNING: This is currently an experimental feature. Interfaces and implementation are likely to change.

Perl now uses the File::Glob implementation of the glob() operator automatically. This avoids using an external csh process and the problems associated with it.

Binary numbers supported

Binary numbers are now supported as literals, in s?printf formats, and oct():

    $answer = 0b101010;
    printf "The answer is: %b\n", oct("0b101010");

Some arrows may be omitted in calls through references

Perl now allows the arrow to be omitted in many constructs involving subroutine calls through references. For example, $foo[10]->('foo') may now be written $foo[10]('foo'). This is rather similar to how the arrow may be omitted from $foo[10]->{'foo'}. Note however, that the arrow is still required for foo(10)->('bar').

exists() is supported on subroutine names

The exists() builtin now works on subroutine names. A subroutine is considered to exist if it has been declared (even if implicitly). See "exists" in perlfunc for examples.

exists() and delete() are supported on array elements

The exists() and delete() builtins now work on simple arrays as well. The behavior is similar to that on hash elements.

exists() can be used to check whether an array element has been initialized. This avoids autovivifying array elements that don't exist. If the array is tied, the EXISTS() method in the corresponding tied package will be invoked.

delete() may be used to remove an element from the array and return it. The array element at that position returns to its unintialized state, so that testing for the same element with exists() will return false. If the element happens to be the one at the end, the size of the array also shrinks up to the highest element that tests true for exists(), or 0 if none such is found. If the array is tied, the DELETE() method in the corresponding tied package will be invoked.

See "exists" in perlfunc and "delete" in perlfunc for examples.

syswrite() ease-of-use

The length argument of syswrite() has become optional.

File and directory handles can be autovivified

Similar to how constructs such as $x->[0] autovivify a reference, handle constructors (open(), opendir(), pipe(), socketpair(), sysopen(), socket(), and accept()) now autovivify a file or directory handle if the handle passed to them is an uninitialized scalar variable. This allows the constructs such as open(my $fh, ...) and open(local $fh,...) to be used to create filehandles that will conveniently be closed automatically when the scope ends, provided there are no other references to them. This largely eliminates the need for typeglobs when opening filehandles that must be passed around, as in the following example:

    sub myopen {
        open my $fh, "@_"
             or die "Can't open '@_': $!";
        return $fh;

        my $f = myopen("</etc/motd");
        print <$f>;
        # $f implicitly closed here

[TODO - this idiom needs more pod penetration]

64-bit support

        NOTE:   The Configure flags -Duselonglong and -Duse64bits
                have been deprecated.  Use -Duse64bitint instead.

Any platform that has 64-bit integers either (a) natively as longs or ints (b) via special compiler flags (c) using long long are able to use "quads" (64-integers) as follows:

  • constants (decimal, hexadecimal, octal, binary) in the code

  • arguments to oct() and hex()

  • arguments to print(), printf() and sprintf() (flag prefixes ll, L, q)

  • printed as such

  • pack() and unpack() "q" and "Q" formats

  • in basic arithmetics: + - * / % (NOTE: operating close to the limits of the integer values may produce surprising results)

  • in bit arithmetics: & | ^ ~ << >> (NOTE: these used to be forced to be 32 bits wide.)

  • vec()

Note that unless you have the case (a) you will have to configure and compile Perl using the -Duse64bitint Configure flag.

There are actually two modes of 64-bitness: the first one is achieved using Configure -Duse64bitint and the second one using Configure -Duse64bitall. The difference is that the first one is minimal and the second one maximal. The first one does only as much as is required to get 64-bit integers into Perl (this may mean, for example, using "long longs") while your memory may still be limited to 2 gigabytes (because your pointers most likely are 32-bit); the second one goes all the way by attempting to switch also longs (and pointers) being 64-bit. This may create an even more binary incompatible Perl than -Duse64bitint: the resulting executable may not run at all in a CPU-bit box, or you may have to reboot/reconfigure/rebuild your operating system to be 64-bit aware.

Natively 64-bit systems like Alpha and Cray need neither -Duse64bitint nor -Duse64bitall.

Last but not least: note that due to Perl's habit of always using floating point numbers the quads are still not true integers. When quads overflow their limits (0...18_446_744_073_709_551_615 unsigned, -9_223_372_036_854_775_808...9_223_372_036_854_775_807 signed), they are silently promoted to floating point numbers, after which they will start losing precision (their lower digits).

Large file support

If you have filesystems that support "large files" (files larger than 2 gigabytes), you may now also be able to create and access them from Perl. You have to use Configure -Duselargefiles. Turning on the large file support turns on also the 64-bit support on many platforms. Beware that unless your filesystem also supports "sparse files" seeking to umpteen petabytes may be unadvisable.

Note that in addition to requiring a proper file system to do large files you may also need to adjust your per-process (or your per-system, or per-process-group, or per-user-group) maximum filesize limits before running Perl scripts that try to handle large files, especially if you intend to write such files.

Finally, in addition to your process/process group maximum filesize limits, you may have quota limits on your filesystems that stop you (your user id or your user group id) from using large files.

Adjusting your process/user/group/file system/operating system limits is outside the scope of Perl core language. For process limits, you may try increasing the limits using your shell's limits/limit/ulimit command before running Perl. The BSD::Resource extension (not included with the standard Perl distribution) may also be of use, it offers the getrlimit/setrlimit interface that can be used to adjust process resource usage limits, including the maximum filesize limit.

Long doubles

In some systems you may be able to use long doubles to enhance the range and precision of your double precision floating point numbers (that is, Perl's numbers). Use Configure -Duselongdouble to enable this support (if it is available).

"more bits"

You can "Configure -Dusemorebits" to turn on both the 64-bit support and the long double support.

Enhanced support for sort() subroutines

Perl subroutines with a prototype of ($$) and XSUBs in general can now be used as sort subroutines. In either case, the two elements to be compared are passed as normal parameters in @_. See "sort" in perlfunc.

For unprototyped sort subroutines, the historical behavior of passing the elements to be compared as the global variables $a and $b remains unchanged.

Better syntax checks on parenthesized unary operators

Expressions such as:

    print defined(&foo,&bar,&baz);
    print uc("foo","bar","baz");

used to be accidentally allowed in earlier versions, and produced unpredictable behaviour. Some produced ancillary warnings when used in this way; others silently did the wrong thing.

The parenthesized forms of most unary operators that expect a single argument now ensure that they are not called with more than one argument, making the cases shown above syntax errors. The usual behaviour of:

    print defined &foo, &bar, &baz;
    print uc "foo", "bar", "baz";
    undef $foo, &bar;

remains unchanged. See perlop.

POSIX character class syntax [: :] supported

For example to match alphabetic characters use /[[:alpha:]]/. See perlre for details.

Improved qw// operator

The qw// operator is now evaluated at compile time into a true list instead of being replaced with a run time call to split(). This removes the confusing misbehaviour of qw// in scalar context, which had inherited that behaviour from split().


    $foo = ($bar) = qw(a b c); print "$foo|$bar\n";

now correctly prints "3|a", instead of "2|a".

pack() format 'Z' supported

The new format type 'Z' is useful for packing and unpacking null-terminated strings. See "pack" in perlfunc.

pack() format modifier '!' supported

The new format type modifier '!' is useful for packing and unpacking native shorts, ints, and longs. See "pack" in perlfunc.

pack() and unpack() support counted strings

The template character '/' can be used to specify a counted string type to be packed or unpacked. See "pack" in perlfunc.

Comments in pack() templates

The '#' character in a template introduces a comment up to end of the line. This facilitates documentation of pack() templates.

$^X variables may now have names longer than one character

Formerly, $^X was synonymous with ${"\cX"}, but $^XY was a syntax error. Now variable names that begin with a control character may be arbitrarily long. However, for compatibility reasons, these variables must be written with explicit braces, as ${^XY} for example. ${^XYZ} is synonymous with ${"\cXYZ"}. Variable names with more than one control character, such as ${^XY^Z}, are illegal.

The old syntax has not changed. As before, `^X' may be either a literal control-X character or the two-character sequence `caret' plus `X'. When braces are omitted, the variable name stops after the control character. Thus "$^XYZ" continues to be synonymous with $^X . "YZ" as before.

As before, lexical variables may not have names beginning with control characters. As before, variables whose names begin with a control character are always forced to be in package `main'. All such variables are reserved for future extensions, except those that begin with ^_, which may be used by user programs and are guaranteed not to acquire special meaning in any future version of Perl.

use attrs implicit in subroutine attributes

Formerly, if you wanted to mark a subroutine as being a method call or as requiring an automatic lock() when it is entered, you had to declare that with a use attrs pragma in the body of the subroutine. That can now be accomplished with declaration syntax, like this:

    sub mymethod : locked method ;
    sub mymethod : locked method {

    sub othermethod :locked :method ;
    sub othermethod :locked :method {

(Note how only the first : is mandatory, and whitespace surrounding the : is optional.)

AutoSplit.pm and SelfLoader.pm have been updated to keep the attributes with the stubs they provide. See attributes.

Regular expression improvements

change#2827,2373,2372,2365,1813,1800,4112,4158,4215,4301 [TODO - Ilya Zakharevich <ilya@math.ohio-state.edu>]

Overloading improvements

change#2150 [TODO - Ilya Zakharevich <ilya@math.ohio-state.edu>]

open() with more than two arguments

[TODO - Ilya Zakharevich <ilya@math.ohio-state.edu>]

Support for interpolating named characters

change#4052 [TODO - Ilya Zakharevich <ilya@math.ohio-state.edu>]

Experimental support for user-hooks in @INC

[TODO - Ken Fox <kfox@ford.com>]

require and do may be overridden

require and do 'file' operations may be overridden locally by importing subroutines of the same name into the current package (or globally by importing them into the CORE::GLOBAL:: namespace). Overriding require will also affect use, provided the override is visible at compile-time. See "Overriding Built-in Functions" in perlsub.

New variable $^C reflects -c switch

$^C has a boolean value that reflects whether perl is being run in compile-only mode (i.e. via the -c switch). Since BEGIN blocks are executed under such conditions, this variable enables perl code to determine whether actions that make sense only during normal running are warranted. See perlvar.

New variable $^V contains Perl version in v5.6.0 format

$^V contains the Perl version number as a string composed of characters whose ordinals match the version numbers, so that it may be used in string comparisons.

See Support for strings represented as a vector of ordinals for an example.

Optional Y2K warnings

If Perl is built with the cpp macro PERL_Y2KWARN defined, it emits optional warnings when concatenating the number 19 with another number.

This behavior must be specifically enabled when running Configure. See INSTALL and README.Y2K.

Significant bug fixes

<HANDLE> on empty files

With $/ set to undef, "slurping" an empty file returns a string of zero length (instead of undef, as it used to) the first time the HANDLE is read after $/ is set to undef. Further reads yield undef.

This means that the following will append "foo" to an empty file (it used to do nothing):

    perl -0777 -pi -e 's/^/foo/' empty_file

The behaviour of:

    perl -pi -e 's/^/foo/' empty_file

is unchanged (it continues to leave the file empty).

eval '...' improvements

Line numbers (as reflected by caller() and most diagnostics) within eval '...' were often incorrect when here documents were involved. This has been corrected.

Lexical lookups for variables appearing in eval '...' within functions that were themselves called within an eval '...' were searching the wrong place for lexicals. The lexical search now correctly ends at the subroutine's block boundary.

Parsing of here documents used to be flawed when they appeared as the replacement expression in eval 's/.../.../e'. This has been fixed.

All compilation errors are true errors

Some "errors" encountered at compile time were by neccessity generated as warnings followed by eventual termination of the program. This enabled more such errors to be reported in a single run, rather than causing a hard stop at the first error that was encountered.

The mechanism for reporting such errors has been reimplemented to queue compile-time errors and report them at the end of the compilation as true errors rather than as warnings. This fixes cases where error messages leaked through in the form of warnings when code was compiled at run time using eval STRING, and also allows such errors to be reliably trapped using __DIE__ hooks.

Automatic flushing of output buffers

fork(), exec(), system(), qx//, and pipe open()s now flush buffers of all files opened for output when the operation was attempted. This mostly eliminates confusing buffering mishaps suffered by users unaware of how Perl internally handles I/O.

Better diagnostics on meaningless filehandle operations

Constructs such as open(<FH>) and close(<FH>) are compile time errors. Attempting to read from filehandles that were opened only for writing will now produce warnings (just as writing to read-only filehandles does).

Where possible, buffered data discarded from duped input filehandle

open(NEW, "<&OLD") now attempts to discard any data that was previously read and buffered in OLD before duping the handle. On platforms where doing this is allowed, the next read operation on NEW will return the same data as the corresponding operation on OLD. Formerly, it would have returned the data from the start of the following disk block instead.

eof() has the same old magic as <>

eof() would return true if no attempt to read from <> had yet been made. eof() has been changed to have a little magic of its own, it now opens the <> files.

system(), backticks and pipe open now reflect exec() failure

On Unix and similar platforms, system(), qx() and open(FOO, "cmd |") etc., are implemented via fork() and exec(). When the underlying exec() fails, earlier versions did not report the error properly, since the exec() happened to be in a different process.

The child process now communicates with the parent about the error in launching the external command, which allows these constructs to return with their usual error value and set $!.

Implicitly closed filehandles are safer

Sometimes implicitly closed filehandles (as when they are localized, and Perl automatically closes them on exiting the scope) could inadvertently set $? or $!. This has been corrected.

(\$) prototype and $foo{a}

An scalar reference prototype now correctly allows a hash or array element in that slot.

Pseudo-hashes work better

Dereferencing some types of reference values in a pseudo-hash, such as $ph->{foo}[1], was accidentally disallowed. This has been corrected.

When applied to a pseudo-hash element, exists() now reports whether the specified value exists, not merely if the key is valid.

delete() now works on pseudo-hashes. When given a pseudo-hash element or slice it deletes the values corresponding to the keys (but not the keys themselves). See "Pseudo-hashes: Using an array as a hash" in perlref.

Pseudo-hash slices with constant keys are now optimized to array lookups at compile-time.

The fields pragma now provides ways to create pseudo-hashes, via fields::new() and fields::phash(). See fields.

goto &sub and AUTOLOAD

The goto &sub construct works correctly when &sub happens to be autoloaded.

-bareword allowed under use integer

The autoquoting of barewords preceded by - did not work in prior versions when the integer pragma was enabled. This has been fixed.

Constructs such as ($a ||= 2) += 1 are now allowed.

sort $coderef @foo allowed

sort() did not accept a subroutine reference as the comparison function in earlier versions. This is now permitted.

Failures in DESTROY()

When code in a destructor threw an exception, it went unnoticed in earlier versions of Perl, unless someone happened to be looking in $@ just after the point the destructor happened to run. Such failures are now visible as warnings when warnings are enabled.

Locale bugs fixed

printf() and sprintf() previously reset the numeric locale back to the default "C" locale. This has been fixed.

Numbers formatted according to the local numeric locale (such as using a decimal comma instead of a decimal dot) caused "isn't numeric" warnings, even while the operations accessing those numbers produced correct results. The warnings are gone.

Memory leaks

The eval 'return sub {...}' construct could sometimes leak memory. This has been fixed.

Operations that aren't filehandle constructors used to leak memory when used on invalid filehandles. This has been fixed.

Constructs that modified @_ could fail to deallocate values in @_ and thus leak memory. This has been corrected.

Spurious subroutine stubs after failed subroutine calls

Perl could sometimes create empty subroutine stubs when a subroutine was not found in the package. Such cases stopped later method lookups from progressing into base packages. This has been corrected.

Consistent numeric conversions

change#3378,3318 [TODO - Ilya Zakharevich <ilya@math.ohio-state.edu>]

Taint failures under -U

When running in unsafe mode, taint violations could sometimes cause silent failures. This has been fixed.

END blocks and the -c switch

Prior versions used to run BEGIN and END blocks when Perl was run in compile-only mode. Since this is typically not the expected behavior, END blocks are not executed anymore when the -c switch is used.

See "CHECK blocks" for how to run things when the compile phase ends.

Potential to leak DATA filehandles

Using the __DATA__ token creates an implicit filehandle to the file that contains the token. It is the program's responsibility to close it when it is done reading from it.

This caveat is now better explained in the documentation. See perldata.

Diagnostics follow STDERR

Diagnostic output now goes to whichever file the STDERR handle is pointing at, instead of always going to the underlying C runtime library's stderr.

Other fixes for better diagnostics

Line numbers are no longer suppressed (under most likely circumstances) during the global destruction phase.

Diagnostics emitted from code running in threads other than the main thread are now accompanied by the thread ID.

Embedded null characters in diagnostics now actually show up. They used to truncate the message in prior versions.

$foo::a and $foo::b are now exempt from "possible typo" warnings only if sort() is encountered in package foo.

Unrecognized alphabetic escapes encountered when parsing quote constructs now generate a warning, since they may take on new semantics in later versions of Perl.

Performance enhancements

Simple sort() using { $a <=> $b } and the like are optimized

Many common sort() operations using a simple inlined block are now optimized for faster performance.

Optimized assignments to lexical variables

Certain operations in the RHS of assignment statements have been optimized to directly set the lexical variable on the LHS, eliminating redundant copying overheads.

Method lookups optimized

[TODO - Chip Salzenberg <chip@perlsupport.com>]

Faster mechanism to invoke XSUBs

change#4044,4125 [TODO - Ilya Zakharevich <ilya@math.ohio-state.edu>]

Perl_malloc() improvements

change#4237 [TODO - Ilya Zakharevich <ilya@math.ohio-state.edu>]

Faster subroutine calls

Minor changes in how subroutine calls are handled internally provide marginal improvements in performance.

Platform specific changes

Additional supported platforms

  • VM/ESA is now supported.

  • Siemens BS2000 is now supported under the POSIX Shell.

  • The Mach CThreads (NEXTSTEP, OPENSTEP) are now supported by the Thread extension.

  • GNU/Hurd is now supported.

  • Rhapsody is now supported.

  • EPOC is is now supported (on Psion 5).


  • Perl now works with djgpp 2.02 (and 2.03 alpha).

  • Environment variable names are not converted to uppercase any more.

  • Wrong exit code from backticks now fixed.

  • This port is still using its own builtin globbing.


[TODO - Ilya Zakharevich <ilya@math.ohio-state.edu>]


[TODO - Charles Bailey <bailey@newman.upenn.edu>]


Site library searches failed to look for ".../site/5.XXX/lib" if ".../site/5.XXXYY/lib" wasn't found. This has been corrected.

When given a pathname that consists only of a drivename, such as A:, opendir() and stat() now use the current working directory for the drive rather than the drive root.

The builtin XSUB functions in the Win32:: namespace are documented. See Win32.

$^X now contains the full path name of the running executable.

A Win32::GetLongPathName() function is provided to complement Win32::GetFullPathName() and Win32::GetShortPathName(). See Win32.

POSIX::uname() is supported.

system(1,...) now returns true process IDs rather than process handles. kill() accepts any real process id, rather than strictly return values from system(1,...).

For better compatibility with Unix, kill(0, $pid) can now be used to test whether a process exists.

The Shell module is supported.

Rudimentary support for building under command.com in Windows 95 has been added.

Scripts are read in binary mode by default to allow ByteLoader (and the filter mechanism in general) to work properly. For compatibility, the DATA filehandle will be set to text mode if a carriage return is detected at the end of the line containing the __END__ or __DATA__ token; if not, the DATA filehandle will be left open in binary mode. Earlier versions always opened the DATA filehandle in text mode.

The glob() operator is implemented via the File::Glob extension, which supports glob syntax of the C shell. This increases the flexibility of the glob() operator, but there may be compatibility issues for programs that relied on the older globbing syntax. If you want to preserve compatibility with the older syntax, you might want to put a use File::DosGlob; in your program. For details and compatibility information, see File::Glob.


New tests


Compatibility tests for sub : attrs vs the older use attrs.


IO constants (SEEK_*, _IO*).


Directory-related IO methods (new, read, close, rewind, tied delete).


INET sockets with multi-homed hosts.


IO poll().


UNIX sockets.


Regression tests for my ($x,@y,%z) : attrs and <sub : attrs>.


File test operators.


Verify operations that access pad objects (lexicals and temporaries).


Verify exists &sub operations.

Modules and Pragmata



While used internally by Perl as a pragma, this module also provides a way to fetch subroutine and variable attributes. See attributes.


The Perl Compiler suite has been extensively reworked for this release.

[TODO - Vishal Bhatia <vishal@gol.com>, Nick Ing-Simmons <nick@ni-s.u-net.com>]


The ByteLoader is a dedicated extension to generate and run Perl bytecode. See ByteLoader.


References can now be used.

The new version also allows a leading underscore in constant names, but disallows a double leading underscore (as in "__LINE__"). Some other names are disallowed or warned against, including BEGIN, END, etc. Some names which were forced into main:: used to fail silently in some cases; now they're fatal (outside of main::) and an optional warning (inside of main::). The ability to detect whether a constant had been set with a given name has been added.

See constant.


change#4052 [TODO - Ilya Zakharevich <ilya@math.ohio-state.edu>]


A Maxdepth setting can be specified to avoid venturing too deeply into deep data structures. See Data::Dumper.

Dumping qr// objects works correctly.


DB is an experimental module that exposes a clean abstraction to Perl's debugging API.


DB_File can now be built with Berkeley DB versions 1, 2 or 3. See ext/DB_File/Changes.


Devel::DProf, a Perl source code profiler has been added. See Devel::DProf and dprofpp.


The Dumpvalue module provides screen dumps of Perl data.


Overall, Benchmark results exhibit lower average error and better timing accuracy.

You can now run tests for n seconds instead of guessing the right number of tests to run: e.g. timethese(-5, ...) will run each code for at least 5 CPU seconds. Zero as the "number of repetitions" means "for at least 3 CPU seconds". The output format has also changed. For example:

   use Benchmark;$x=3;timethese(-5,{a=>sub{$x*$x},b=>sub{$x**2}})

will now output something like this:

   Benchmark: running a, b, each for at least 5 CPU seconds...
            a:  5 wallclock secs ( 5.77 usr +  0.00 sys =  5.77 CPU) @ 200551.91/s (n=1156516)
            b:  4 wallclock secs ( 5.00 usr +  0.02 sys =  5.02 CPU) @ 159605.18/s (n=800686)

New features: "each for at least N CPU seconds...", "wallclock secs", and the "@ operations/CPU second (n=operations)".

timethese() now returns a reference to a hash of Benchmark objects containing the test results, keyed on the names of the tests.

timethis() now returns the iterations field in the Benchmark result object instead of 0.

timethese(), timethis(), and the new cmpthese() (see below) can also take a format specifier of 'none' to suppress output.

A new function countit() is just like timeit() except that it takes a TIME instead of a COUNT.

A new function cmpthese() prints a chart comparing the results of each test returned from a timethese() call. For each possible pair of tests, the percentage speed difference (iters/sec or seconds/iter) is shown.

For other details, see Benchmark.


The Devel::Peek module provides access to the internal representation of Perl variables and data. It is a data debugging tool for the XS programmer.


$PERL_VERSION now stands for $^V (a string value) rather than for $] (a numeric value).


change#4135, also needs docs in module pod [TODO - Ilya Zakharevich <ilya@math.ohio-state.edu>]


More Fcntl constants added: F_SETLK64, F_SETLKW64, O_LARGEFILE for large file (more than 4GB) access Note that the O_LARGEFILE is automatically/transparently added to sysopen() flags if large file support has been configured), Free/Net/OpenBSD locking behaviour flags F_FLOCK, F_POSIX, Linux F_SHLCK, and O_ACCMODE: the combined mask of O_RDONLY, O_WRONLY, and O_RDWR. The seek()/sysseek() constants SEEK_SET, SEEK_CUR, and SEEK_END are available via the :seek tag. The chmod()/stat() S_IF* constants and S_IS* functions are available via the :mode tag.


A compare_text() function has been added, which allows custom comparison functions. See File::Compare.


File::Find now works correctly when the wanted() function is either autoloaded or is a symbolic reference.

A bug that caused File::Find to lose track of the working directory when pruning top-level directories has been fixed.

File::Find now also supports several other options to control its behavior. It can follow symbolic links if the follow option is specified. Enabling the no_chdir option will make File::Find skip changing the current directory when walking directories. The untaint flag can be useful when running with taint checks enabled.

See File::Find.


This extension implements BSD-style file globbing. By default, it will also be used for the internal implementation of the glob() operator. See File::Glob.


New methods have been added to the File::Spec module: devnull() returns the name of the null device (/dev/null on Unix) and tmpdir() the name of the temp directory (normally /tmp on Unix). There are now also methods to convert between absolute and relative filenames: abs2rel() and rel2abs(). For compatibility with operating systems that specify volume names in file paths, the splitpath(), splitdir(), and catdir() methods have been added.


The new File::Spec::Functions modules provides a function interface to the File::Spec module. Allows shorthand

    $fullname = catfile($dir1, $dir2, $file);

instead of

    $fullname = File::Spec->catfile($dir1, $dir2, $file);

Getopt::Long licensing has changed to allow the Perl Artistic License as well as the GPL. It used to be GPL only, which got in the way of non-GPL applications that wanted to use Getopt::Long.

Getopt::Long encourages the use of Pod::Usage to produce help messages. For example:

    use Getopt::Long;
    use Pod::Usage;
    my $man = 0;
    my $help = 0;
    GetOptions('help|?' => \$help, man => \$man) or pod2usage(2);
    pod2usage(1) if $help;
    pod2usage(-exitstatus => 0, -verbose => 2) if $man;


    =head1 NAME

    sample - Using GetOpt::Long and Pod::Usage

    =head1 SYNOPSIS

    sample [options] [file ...]

       -help            brief help message
       -man             full documentation

    =head1 OPTIONS

    =over 8

    =item B<-help>

    Print a brief help message and exits.

    =item B<-man>

    Prints the manual page and exits.


    =head1 DESCRIPTION

    B<This program> will read the given input file(s) and do someting
    useful with the contents thereof.


See Pod::Usage for details.

A bug that prevented the non-option call-back <> from being specified as the first argument has been fixed.

To specify the characters < and > as option starters, use ><. Note, however, that changing option starters is strongly deprecated.


write() and syswrite() will now accept a single-argument form of the call, for consistency with Perl's syswrite().

You can now create a TCP-based IO::Socket::INET without forcing a connect attempt. This allows you to configure its options (like making it non-blocking) and then call connect() manually.

A bug that prevented the IO::Socket::protocol() accessor from ever returning the correct value has been corrected.


Java Perl Lingo is now distributed with Perl. See jpl/README for more information.


use lib now weeds out any trailing duplicate entries. no lib removes all named entries.


The bitwise operations <<, >>, &, |, and ~ are now supported on bigints.


The accessor methods Re, Im, arg, abs, rho, and theta can now also act as mutators (accessor $z->Re(), mutator $z->Re(3)).


A little bit of radial trigonometry (cylindrical and spherical), radial coordinate conversions, and the great circle distance were added.

Pod::Parser, Pod::InputObjects

Pod::Parser is a base class for parsing and selecting sections of pod documentation from an input stream. This module takes care of identifying pod paragraphs and commands in the input and hands off the parsed paragraphs and commands to user-defined methods which are free to interpret or translate them as they see fit.

Pod::InputObjects defines some input objects needed by Pod::Parser, and for advanced users of Pod::Parser that need more about a command besides its name and text.

As of release 5.6 of Perl, Pod::Parser is now the officially sanctioned "base parser code" recommended for use by all pod2xxx translators. Pod::Text (pod2text) and Pod::Man (pod2man) have already been converted to use Pod::Parser and efforts to convert Pod::HTML (pod2html) are already underway. For any questions or comments about pod parsing and translating issues and utilities, please use the pod-people@perl.org mailing list.

For further information, please see Pod::Parser and Pod::InputObjects.

Pod::Checker, podchecker

This utility checks pod files for correct syntax, according to perlpod. Obvious errors are flagged as such, while warnings are printed for mistakes that can be handled gracefully. The checklist is not complete yet. See Pod::Checker.

Pod::ParseUtils, Pod::Find

These modules provide a set of gizmos that are useful mainly for pod translators. Pod::Find traverses directory structures and returns found pod files, along with their canonical names (like File::Spec::Unix). Pod::ParseUtils contains Pod::List (useful for storing pod list information), Pod::Hyperlink (for parsing the contents of L>< sequences) and Pod::Cache (for caching information about pod files, e.g. link nodes).

Pod::Select, podselect

Pod::Select is a subclass of Pod::Parser which provides a function named "podselect()" to filter out user-specified sections of raw pod documentation from an input stream. podselect is a script that provides access to Pod::Select from other scripts to be used as a filter. See Pod::Select.

Pod::Usage, pod2usage

Pod::Usage provides the function "pod2usage()" to print usage messages for a Perl script based on its embedded pod documentation. The pod2usage() function is generally useful to all script authors since it lets them write and maintain a single source (the pods) for documentation, thus removing the need to create and maintain redundant usage message text consisting of information already in the pods.

There is also a pod2usage script which can be used from other kinds of scripts to print usage messages from pods (even for non-Perl scripts with pods embedded in comments).

For details and examples, please see Pod::Usage.

Pod::Text and Pod::Man

[TODO - Russ Allbery <rra@stanford.edu>]


An EXISTS method has been added to this module (and sdbm_exists() has been added to the underlying sdbm library), so one can now call exists on an SDBM_File tied hash and get the correct result, rather than a runtime error.

A bug that may have caused data loss when more than one disk block happens to be read from the database in a single FETCH() has been fixed.


Sys::Syslog now uses XSUBs to access facilities from syslog.h so it no longer requires syslog.ph to exist.


Sys::Hostname now uses XSUBs to call the C library's gethostname() or uname() if they exist.


The timelocal() and timegm() functions used to silently return bogus results when the date fell outside the machine's integer range. They now consistently croak() if the date falls in an unsupported range.


The error return value in list context has been changed for all functions that return a list of values. Previously these functions returned a list with a single element undef if an error occurred. Now these functions return the empty list in these situations. This applies to the following functions:


The remaining functions are unchanged and continue to return undef on error even in list context.

The Win32::SetLastError(ERROR) function has been added as a complement to the Win32::GetLastError() function.

The new Win32::GetFullPathName(FILENAME) returns the full absolute pathname for FILENAME in scalar context. In list context it returns a two-element list containing the fully qualified directory name and the filename. See Win32.

DBM Filters

A new feature called "DBM Filters" has been added to all the DBM modules--DB_File, GDBM_File, NDBM_File, ODBM_File, and SDBM_File. DBM Filters add four new methods to each DBM module:


These can be used to filter key-value pairs before the pairs are written to the database or just after they are read from the database. See perldbmfilter for further information.


use attrs is now obsolete, and is only provided for backward-compatibility. It's been replaced by the sub : attributes syntax. See "Subroutine Attributes" in perlsub and attributes.

use utf8 to enable UTF-8 and Unicode support.

Lexical warnings pragma, use warnings;, to control optional warnings. See perllexwarn.

use filetest to control the behaviour of filetests (-r -w ...). Currently only one subpragma implemented, "use filetest 'access';", that uses access(2) or equivalent to check permissions instead of using stat(2) as usual. This matters in filesystems where there are ACLs (access control lists): the stat(2) might lie, but access(2) knows better.

Utility Changes


[TODO - Kurt Starsinic <kstar@chapin.edu>]


perlcc now supports the C and Bytecode backends. By default, it generates output from the simple C backend rather than the optimized C backend.

Support for non-Unix platforms has been improved.


change#4232 [TODO - Ilya Zakharevich <ilya@math.ohio-state.edu>]

Documentation Changes


The official list of public Perl API functions.


An introduction to using the Perl Compiler suite.


An introduction to writing Perl source filters.


Some guidelines for hacking the Perl source code.


A list of internal functions in the Perl source code. (List is currently empty.)


A tutorial on using open() effectively.


A tutorial that introduces the essentials of references.


A tutorial for beginners on object-oriented Perl.


A tutorial on managing class data for object modules.


An introduction to Unicode support features in Perl.

New or Changed Diagnostics

"%s" variable %s masks earlier declaration in same %s

(W) A "my" or "our" variable has been redeclared in the current scope or statement, effectively eliminating all access to the previous instance. This is almost always a typographical error. Note that the earlier variable will still exist until the end of the scope or until all closure referents to it are destroyed.

"my sub" not yet implemented

(F) Lexically scoped subroutines are not yet implemented. Don't try that yet.

"our" variable %s redeclared

(W) You seem to have already declared the same global once before in the current lexical scope.

'!' allowed only after types %s

(F) The '!' is allowed in pack() and unpack() only after certain types. See "pack" in perlfunc.

/ cannot take a count

(F) You had an unpack template indicating a counted-length string, but you have also specified an explicit size for the string. See "pack" in perlfunc.

/ must be followed by a, A or Z

(F) You had an unpack template indicating a counted-length string, which must be followed by one of the letters a, A or Z to indicate what sort of string is to be unpacked. See "pack" in perlfunc.

/ must be followed by a*, A* or Z*

(F) You had a pack template indicating a counted-length string, Currently the only things that can have their length counted are a*, A* or Z*. See "pack" in perlfunc.

/ must follow a numeric type

(F) You had an unpack template that contained a '#', but this did not follow some numeric unpack specification. See "pack" in perlfunc.

/%s/: Unrecognized escape \\%c passed through

(W) You used a backslash-character combination which is not recognized by Perl. This combination appears in an interpolated variable or a '-delimited regular expression. The character was understood literally.

/%s/: Unrecognized escape \\%c in character class passed through

(W) You used a backslash-character combination which is not recognized by Perl inside character classes. The character was understood literally.

/%s/ should probably be written as "%s"

(W) You have used a pattern where Perl expected to find a string, as in the first argument to join. Perl will treat the true or false result of matching the pattern against $_ as the string, which is probably not what you had in mind.

%s() called too early to check prototype

(W) You've called a function that has a prototype before the parser saw a definition or declaration for it, and Perl could not check that the call conforms to the prototype. You need to either add an early prototype declaration for the subroutine in question, or move the subroutine definition ahead of the call to get proper prototype checking. Alternatively, if you are certain that you're calling the function correctly, you may put an ampersand before the name to avoid the warning. See perlsub.

%s argument is not a HASH or ARRAY element

(F) The argument to exists() must be a hash or array element, such as:

%s argument is not a HASH or ARRAY element or slice

(F) The argument to delete() must be either a hash or array element, such as:


or a hash or array slice, such as:

    @foo[$bar, $baz, $xyzzy]
    @{$ref->[12]}{"susie", "queue"}
%s argument is not a subroutine name

(F) The argument to exists() for exists &sub must be a subroutine name, and not a subroutine call. exists &sub() will generate this error.

%s package attribute may clash with future reserved word: %s

(W) A lowercase attribute name was used that had a package-specific handler. That name might have a meaning to Perl itself some day, even though it doesn't yet. Perhaps you should use a mixed-case attribute name, instead. See attributes.

(in cleanup) %s

(W) This prefix usually indicates that a DESTROY() method raised the indicated exception. Since destructors are usually called by the system at arbitrary points during execution, and often a vast number of times, the warning is issued only once for any number of failures that would otherwise result in the same message being repeated.

Failure of user callbacks dispatched using the G_KEEPERR flag could also result in this warning. See "G_KEEPERR" in perlcall.

<> should be quotes

(F) You wrote require <file> when you should have written require 'file'.

Attempt to join self

(F) You tried to join a thread from within itself, which is an impossible task. You may be joining the wrong thread, or you may need to move the join() to some other thread.

Bad evalled substitution pattern

(F) You've used the /e switch to evaluate the replacement for a substitution, but perl found a syntax error in the code to evaluate, most likely an unexpected right brace '}'.

Bad realloc() ignored

(S) An internal routine called realloc() on something that had never been malloc()ed in the first place. Mandatory, but can be disabled by setting environment variable PERL_BADFREE to 1.

Bareword found in conditional

(W) The compiler found a bareword where it expected a conditional, which often indicates that an || or && was parsed as part of the last argument of the previous construct, for example:

    open FOO || die;

It may also indicate a misspelled constant that has been interpreted as a bareword:

    use constant TYPO => 1;
    if (TYOP) { print "foo" }

The strict pragma is useful in avoiding such errors.

Binary number > 0b11111111111111111111111111111111 non-portable

(W) The binary number you specified is larger than 2**32-1 (4294967295) and therefore non-portable between systems. See perlport for more on portability concerns.

Bit vector size > 32 non-portable

(W) Using bit vector sizes larger than 32 is non-portable.

Buffer overflow in prime_env_iter: %s

(W) A warning peculiar to VMS. While Perl was preparing to iterate over %ENV, it encountered a logical name or symbol definition which was too long, so it was truncated to the string shown.

Can't check filesystem of script "%s"

(P) For some reason you can't check the filesystem of the script for nosuid.

Can't declare class for non-scalar %s in "%s"

(S) Currently, only scalar variables can declared with a specific class qualifier in a "my" or "our" declaration. The semantics may be extended for other types of variables in future.

Can't declare %s in "%s"

(F) Only scalar, array, and hash variables may be declared as "my" or "our" variables. They must have ordinary identifiers as names.

Can't ignore signal CHLD, forcing to default

(W) Perl has detected that it is being run with the SIGCHLD signal (sometimes known as SIGCLD) disabled. Since disabling this signal will interfere with proper determination of exit status of child processes, Perl has reset the signal to its default value. This situation typically indicates that the parent program under which Perl may be running (e.g. cron) is being very careless.

Can't modify non-lvalue subroutine call

(F) Subroutines meant to be used in lvalue context should be declared as such, see "Lvalue subroutines" in perlsub.

Can't read CRTL environ

(S) A warning peculiar to VMS. Perl tried to read an element of %ENV from the CRTL's internal environment array and discovered the array was missing. You need to figure out where your CRTL misplaced its environ or define PERL_ENV_TABLES (see perlvms) so that environ is not searched.

Can't remove %s: %s, skipping file

(S) You requested an inplace edit without creating a backup file. Perl was unable to remove the original file to replace it with the modified file. The file was left unmodified.

Can't return %s from lvalue subroutine

(F) Perl detected an attempt to return illegal lvalues (such as temporary or readonly values) from a subroutine used as an lvalue. This is not allowed.

Can't weaken a nonreference

(F) You attempted to weaken something that was not a reference. Only references can be weakened.

Character class [:%s:] unknown

(F) The class in the character class [: :] syntax is unknown. See perlre.

Character class syntax [%s] belongs inside character classes

(W) The character class constructs [: :], [= =], and [. .] go inside character classes, the [] are part of the construct, for example: /[012[:alpha:]345]/. Note that [= =] and [. .] are not currently implemented; they are simply placeholders for future extensions.

Constant is not %s reference

(F) A constant value (perhaps declared using the use constant pragma) is being dereferenced, but it amounts to the wrong type of reference. The message indicates the type of reference that was expected. This usually indicates a syntax error in dereferencing the constant value. See "Constant Functions" in perlsub and constant.

constant(%s): %%^H is not localized

(F) When setting compile-time-lexicalized hash %^H one should set the corresponding bit of $^H as well.

constant(%s): %s

(F) Compile-time-substitutions (such as overloaded constants and character names) were not correctly set up.

defined(@array) is deprecated

(D) defined() is not usually useful on arrays because it checks for an undefined scalar value. If you want to see if the array is empty, just use if (@array) { # not empty } for example.

defined(%hash) is deprecated

(D) defined() is not usually useful on hashes because it checks for an undefined scalar value. If you want to see if the hash is empty, just use if (%hash) { # not empty } for example.

Did not produce a valid header

See Server error.

Did you mean "local" instead of "our"?

(W) Remember that "our" does not localize the declared global variable. You have declared it again in the same lexical scope, which seems superfluous.

Document contains no data

See Server error.

entering effective %s failed

(F) While under the use filetest pragma, switching the real and effective uids or gids failed.

false [] range "%s" in regexp

(W) A character class range must start and end at a literal character, not another character class like \d or [:alpha:]. The "-" in your false range is interpreted as a literal "-". Consider quoting the "-", "\-". See perlre.

Filehandle %s opened only for output

(W) You tried to read from a filehandle opened only for writing. If you intended it to be a read/write filehandle, you needed to open it with "+<" or "+>" or "+>>" instead of with "<" or nothing. If you intended only to read from the file, use "<". See "open" in perlfunc.

flock() on closed filehandle %s

(W) The filehandle you're attempting to flock() got itself closed some time before now. Check your logic flow. flock() operates on filehandles. Are you attempting to call flock() on a dirhandle by the same name?

Global symbol "%s" requires explicit package name

(F) You've said "use strict vars", which indicates that all variables must either be lexically scoped (using "my"), declared beforehand using "our", or explicitly qualified to say which package the global variable is in (using "::").

Hexadecimal number > 0xffffffff non-portable

(W) The hexadecimal number you specified is larger than 2**32-1 (4294967295) and therefore non-portable between systems. See perlport for more on portability concerns.

Ill-formed CRTL environ value "%s"

(W) A warning peculiar to VMS. Perl tried to read the CRTL's internal environ array, and encountered an element without the = delimiter used to spearate keys from values. The element is ignored.

Ill-formed message in prime_env_iter: |%s|

(W) A warning peculiar to VMS. Perl tried to read a logical name or CLI symbol definition when preparing to iterate over %ENV, and didn't see the expected delimiter between key and value, so the line was ignored.

Illegal binary digit %s

(F) You used a digit other than 0 or 1 in a binary number.

Illegal binary digit %s ignored

(W) You may have tried to use a digit other than 0 or 1 in a binary number. Interpretation of the binary number stopped before the offending digit.

Illegal number of bits in vec

(F) The number of bits in vec() (the third argument) must be a power of two from 1 to 32 (or 64, if your platform supports that).

Integer overflow in %s number

(W) The hexadecimal, octal or binary number you have specified either as a literal or as an argument to hex() or oct() is too big for your architecture, and has been converted to a floating point number. On a 32-bit architecture the largest hexadecimal, octal or binary number representable without overflow is 0xFFFFFFFF, 037777777777, or 0b11111111111111111111111111111111 respectively. Note that Perl transparently promotes all numbers to a floating point representation internally--subject to loss of precision errors in subsequent operations.

Invalid %s attribute: %s

The indicated attribute for a subroutine or variable was not recognized by Perl or by a user-supplied handler. See attributes.

Invalid %s attributes: %s

The indicated attributes for a subroutine or variable were not recognized by Perl or by a user-supplied handler. See attributes.

invalid [] range "%s" in regexp

The offending range is now explicitly displayed.

Invalid separator character %s in attribute list

(F) Something other than a colon or whitespace was seen between the elements of an attribute list. If the previous attribute had a parenthesised parameter list, perhaps that list was terminated too soon. See attributes.

Invalid separator character %s in subroutine attribute list

(F) Something other than a colon or whitespace was seen between the elements of a subroutine attribute list. If the previous attribute had a parenthesised parameter list, perhaps that list was terminated too soon.

leaving effective %s failed

(F) While under the use filetest pragma, switching the real and effective uids or gids failed.

Lvalue subs returning %s not implemented yet

(F) Due to limitations in the current implementation, array and hash values cannot be returned in subroutines used in lvalue context. See "Lvalue subroutines" in perlsub.

Method %s not permitted

See Server error.

Missing %sbrace%s on \N{}

(F) Wrong syntax of character name literal \N{charname} within double-quotish context.

Missing command in piped open

(W) You used the open(FH, "| command") or open(FH, "command |") construction, but the command was missing or blank.

Missing name in "my sub"

(F) The reserved syntax for lexically scoped subroutines requires that they have a name with which they can be found.

No %s specified for -%c

(F) The indicated command line switch needs a mandatory argument, but you haven't specified one.

No package name allowed for variable %s in "our"

(F) Fully qualified variable names are not allowed in "our" declarations, because that doesn't make much sense under existing semantics. Such syntax is reserved for future extensions.

No space allowed after -%c

(F) The argument to the indicated command line switch must follow immediately after the switch, without intervening spaces.

no UTC offset information; assuming local time is UTC

(S) A warning peculiar to VMS. Perl was unable to find the local timezone offset, so it's assuming that local system time is equivalent to UTC. If it's not, define the logical name SYS$TIMEZONE_DIFFERENTIAL to translate to the number of seconds which need to be added to UTC to get local time.

Octal number > 037777777777 non-portable

(W) The octal number you specified is larger than 2**32-1 (4294967295) and therefore non-portable between systems. See perlport for more on portability concerns.

See also perlport for writing portable code.

panic: del_backref

(P) Failed an internal consistency check while trying to reset a weak reference.

panic: kid popen errno read

(F) forked child returned an incomprehensible message about its errno.

panic: magic_killbackrefs

(P) Failed an internal consistency check while trying to reset all weak references to an object.

Parentheses missing around "%s" list

(W) You said something like

    my $foo, $bar = @_;

when you meant

    my ($foo, $bar) = @_;

Remember that "my", "our", and "local" bind tighter than comma.

Possible Y2K bug: %s

(W) You are concatenating the number 19 with another number, which could be a potential Year 2000 problem.

Premature end of script headers

See Server error.

Repeat count in pack overflows

(F) You can't specify a repeat count so large that it overflows your signed integers. See "pack" in perlfunc.

Repeat count in unpack overflows

(F) You can't specify a repeat count so large that it overflows your signed integers. See "unpack" in perlfunc.

realloc() of freed memory ignored

(S) An internal routine called realloc() on something that had already been freed.

Reference is already weak

(W) You have attempted to weaken a reference that is already weak. Doing so has no effect.

setpgrp can't take arguments

(F) Your system has the setpgrp() from BSD 4.2, which takes no arguments, unlike POSIX setpgid(), which takes a process ID and process group ID.

Strange *+?{} on zero-length expression

(W) You applied a regular expression quantifier in a place where it makes no sense, such as on a zero-width assertion. Try putting the quantifier inside the assertion instead. For example, the way to match "abc" provided that it is followed by three repetitions of "xyz" is /abc(?=(?:xyz){3})/, not /abc(?=xyz){3}/.

switching effective %s is not implemented

(F) While under the use filetest pragma, we cannot switch the real and effective uids or gids.

This Perl can't reset CRTL environ elements (%s)
This Perl can't set CRTL environ elements (%s=%s)

(W) Warnings peculiar to VMS. You tried to change or delete an element of the CRTL's internal environ array, but your copy of Perl wasn't built with a CRTL that contained the setenv() function. You'll need to rebuild Perl with a CRTL that does, or redefine PERL_ENV_TABLES (see perlvms) so that the environ array isn't the target of the change to %ENV which produced the warning.

Unknown open() mode '%s'

(F) The second argument of 3-argument open() is not among the list of valid modes: <, >, >>, +<, +>, +>>, -|, |-.

Unknown process %x sent message to prime_env_iter: %s

(P) An error peculiar to VMS. Perl was reading values for %ENV before iterating over it, and someone else stuck a message in the stream of data Perl expected. Someone's very confused, or perhaps trying to subvert Perl's population of %ENV for nefarious purposes.

Unrecognized escape \\%c passed through

(W) You used a backslash-character combination which is not recognized by Perl. The character was understood literally.

Unterminated attribute parameter in attribute list

(F) The lexer saw an opening (left) parenthesis character while parsing an attribute list, but the matching closing (right) parenthesis character was not found. You may need to add (or remove) a backslash character to get your parentheses to balance. See attributes.

Unterminated attribute list

(F) The lexer found something other than a simple identifier at the start of an attribute, and it wasn't a semicolon or the start of a block. Perhaps you terminated the parameter list of the previous attribute too soon. See attributes.

Unterminated attribute parameter in subroutine attribute list

(F) The lexer saw an opening (left) parenthesis character while parsing a subroutine attribute list, but the matching closing (right) parenthesis character was not found. You may need to add (or remove) a backslash character to get your parentheses to balance.

Unterminated subroutine attribute list

(F) The lexer found something other than a simple identifier at the start of a subroutine attribute, and it wasn't a semicolon or the start of a block. Perhaps you terminated the parameter list of the previous attribute too soon.

Value of CLI symbol "%s" too long

(W) A warning peculiar to VMS. Perl tried to read the value of an %ENV element from a CLI symbol table, and found a resultant string longer than 1024 characters. The return value has been truncated to 1024 characters.

Version number must be a constant number

(P) The attempt to translate a use Module n.n LIST statement into its equivalent BEGIN block found an internal inconsistency with the version number.

Obsolete Diagnostics

Character class syntax [: :] is reserved for future extensions

(W) Within regular expression character classes ([]) the syntax beginning with "[:" and ending with ":]" is reserved for future extensions. If you need to represent those character sequences inside a regular expression character class, just quote the square brackets with the backslash: "\[:" and ":\]".

Ill-formed logical name |%s| in prime_env_iter

(W) A warning peculiar to VMS. A logical name was encountered when preparing to iterate over %ENV which violates the syntactic rules governing logical names. Because it cannot be translated normally, it is skipped, and will not appear in %ENV. This may be a benign occurrence, as some software packages might directly modify logical name tables and introduce nonstandard names, or it may indicate that a logical name table has been corrupted.

Probable precedence problem on %s

(W) The compiler found a bareword where it expected a conditional, which often indicates that an || or && was parsed as part of the last argument of the previous construct, for example:

    open FOO || die;
regexp too big

(F) The current implementation of regular expressions uses shorts as address offsets within a string. Unfortunately this means that if the regular expression compiles to longer than 32767, it'll blow up. Usually when you want a regular expression this big, there is a better way to do it with multiple statements. See perlre.

Use of "$$<digit>" to mean "${$}<digit>" is deprecated

(D) Perl versions before 5.004 misinterpreted any type marker followed by "$" and a digit. For example, "$$0" was incorrectly taken to mean "${$}0" instead of "${$0}". This bug is (mostly) fixed in Perl 5.004.

However, the developers of Perl 5.004 could not fix this bug completely, because at least two widely-used modules depend on the old meaning of "$$0" in a string. So Perl 5.004 still interprets "$$<digit>" in the old (broken) way inside strings; but it generates this message as a warning. And in Perl 5.005, this special treatment will cease.


If you find what you think is a bug, you might check the articles recently posted to the comp.lang.perl.misc newsgroup. There may also be information at http://www.perl.com/perl/, the Perl Home Page.

If you believe you have an unreported bug, please run the perlbug program included with your release. Make sure to trim your bug down to a tiny but sufficient test case. Your bug report, along with the output of perl -V, will be sent off to perlbug@perl.com to be analysed by the Perl porting team.


The Changes file for exhaustive details on what changed.

The INSTALL file for how to build Perl.

The README file for general stuff.

The Artistic and Copying files for copyright information.


Written by Gurusamy Sarathy <gsar@activestate.com>, with many contributions from The Perl Porters.

Send omissions or corrections to <perlbug@perl.com>.

4 POD Errors

The following errors were encountered while parsing the POD:

Around line 61:

You forgot a '=back' before '=head2'

Around line 88:

'=item' outside of any '=over'

Around line 202:

You forgot a '=back' before '=head2'

Around line 214:

=back without =over