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Gurusamy Sarathy


perldelta - what's new for perl5.005


This document describes differences between the 5.004 release and this one.

[XXX this needs more verbose summaries of the sub topics, instead of just the "See foo." Scheduled for a second iteration. GSAR]

About the new versioning system

Incompatible Changes

WARNING: This version is not binary compatible with Perl 5.004.

Starting with Perl 5.004_50 there were many deep and far-reaching changes to the language internals. If you have dynamically loaded extensions that you built under perl 5.003 or 5.004, you can continue to use them with 5.004, but you will need to rebuild and reinstall those extensions to use them 5.005. See INSTALL for detailed instructions on how to upgrade.

Default installation structure has changed

The new Configure defaults are designed to allow a smooth upgrade from 5.004 to 5.005, but you should read INSTALL for a detailed discussion of the changes in order to adapt them to your system.

Perl Source Compatibility

When none of the experimental features are enabled, there should be no user-visible Perl source compatibility issue.

If threads are enabled, then some caveats apply. @_ and $_ become lexical variables. The effect of this should be largely transparent to the user, but there are some boundary conditions under which user will need to be aware of the issues. [XXX Add e.g. here.]

Some new keywords have been introduced. These are generally expected to have very little impact on compatibility. See "New INIT keyword", "New lock keyword", and "New qr// operator".

Certain barewords are now reserved. Use of these will provoke a warning if you have asked for them with the -w switch. See "our is now a reserved word".

C Source Compatibility

Core sources now require ANSI C compiler
Enabling threads has source compatibility issues

Binary Compatibility

This version is NOT binary compatible with older versions. All extensions will need to be recompiled.

Security fixes may affect compatibility

A few taint leaks and taint omissions have been corrected. This may lead to "failure" of scripts that used to work with older versions. Compiling with -DINCOMPLETE_TAINTS provides a perl with minimal amounts of changes to the tainting behavior. But note that the resulting perl will have known insecurities.

Oneliners with the -e switch do not create temporary files anymore.

Relaxed new mandatory warnings introduced in 5.004

Many new warnings that were introduced in 5.004 have been made optional. Some of these warnings are still present, but perl's new features make them less often a problem. See "New Diagnostics".


Perl has a new Social Contract for contributors. See Porting/Contract.

The license included in much of the Perl documentation has changed. [XXX See where?]

Core Changes


WARNING: Threading is considered an experimental feature. Details of the implementation may change without notice. There are known limitations and and some bugs.

See README.threads.


WARNING: The Compiler and related tools are considered experimental. Features may change without notice, and there are known limitations and bugs.

The Compiler produces three different types of transformations of a perl program. The C backend generates C code that captures perl's state just before execution begins. It eliminates the compile-time overheads of the regular perl interpreter, but the run-time performance remains comparatively the same. The CC backend generates optimized C code equivivalent to the code path at run-time. The CC backend has greater potential for big optimizations, but only a few optimizations are implemented currently. The Bytecode backend generates a platform independent bytecode representation of the interpreter's state just before execution. Thus, the Bytecode back end also eliminates much of the compilation overhead of the interpreter.

The compiler comes with several valuable utilities.

B::Lint is an experimental module to detect and warn about suspicious code, especially the cases that the -w switch does not detect.

B::Deparse can be used to demystify perl code, and understand how perl optimizes certain constructs.

B::Xref generates cross reference reports of all definition and use of variables, subroutines and formats in a program.

B::Showlex show the lexical variables used by a subroutine or file at a glance.

perlcc is a simple frontend for compiling perl.

See ext/B/README.

Regular Expressions

See perlre and perlop.

Improved malloc()

See banner at the beginning of malloc.c for details.

Quicksort is internally implemented

See perlfunc/sort.

Reliable signals

Two kinds.

Via Thread::Signal.

Via switched runtime op loop. [XXX Not yet available.]

Reliable stack pointers

The internals now reallocate the perl stack only at predictable times. In particular, magic calls never trigger reallocations of the stack, because all reentrancy of the runtime is handled using a "stack of stacks". This should improve reliability of cached stack pointers in the internals and in XSUBs.

Behavior of local() on composites is now well-defined

See "local" in perlfunc.

%! is transparently tied to the Errno module

See perlvar, and Errno.

Pseudo-hashes are supported

See perlref.

EXPR foreach EXPR is supported

See perlsyn.

Slice notation on glob elements is supported

[XXX See what?]

Keywords can be globally overridden

See perlsub.

$^E is meaningful on Win32

See perlvar.

foreach (1..1000000) optimized

foreach (1..1000000) is now optimized into a counting loop. It does not try to allocate a 1000000-size list anymore.

Foo:: can be used as implicitly quoted package name

[XXX See what?]

exists $Foo::{Bar::} tests existence of a package

[XXX See what?]

Better locale support

See perllocale.

Experimental support for 64-bit platforms

Perl5 has always had 64-bit support on systems with 64-bit longs. Starting with 5.005, the beginnings of experimental support for systems with 32-bit long and 64-bit 'long long' integers has been added. If you add -DUSE_LONG_LONG to your ccflags in config.sh (or manually define it in perl.h) then perl will be built with 'long long' support. There will be many compiler warnings, and the resultant perl may not work on all systems. There are many other issues related to third-party extensions and libraries. This option exists to allow people to work on those issues.

prototype() returns useful results on builtins

See "prototype" in perlfunc.

Re-blessing in DESTROY() supported for chaining DESTROY() methods

See "Destructors" in perlobj.

All printf format conversions are handled internally

See "printf" in perlfunc.

New INIT keyword

INIT subs are like BEGIN and END, but they get run just before the perl runtime begins execution. e.g., the Perl Compiler makes use of INIT blocks to initialize and resolve pointers to XSUBs.

[XXX Needs to be documented in perlsub or perlmod.]

New lock keyword

The lock keyword is the fundamental synchronization primitive in threaded perl. When threads are not enabled, it is currently a noop.

To minimize impact on source compatibility this keyword is "weak", i.e., any user-defined subroutine of the same name overrides it, unless a use Thread has been seen.

New qr// operator

The qr// operator, which is syntactically similar to the other quote-like operators, is used to create compiled regular expressions. This compiled form can now be explicitly passed around in variables, and interpolated in other regular expressions. See perlop and perlre.

our is now a reserved word

Tied arrays are now fully supported

See Tie::Array.

Tied handles support is better

Several missing hooks have been added. There is also a new base class for TIEARRAY implementations. See Tie::Array.

Supported Platforms

Configure has many incremental improvements. Site-wide policy for building perl can now be made persistent, via Policy.sh. Configure also records the command-line arguments used in config.sh.

New Platforms

BeOS is now supported. See README.beos.

DOS is now supported under the DJGPP tools. See README.dos.

Changes in existing support

Win32 support has been vastly enhanced. Support for Perl Object, a C++ encapsulation of Perl. GCC and EGCS are now supported on Win32. [XXX Perl Object needs a big explanation elsewhere, and a pointer to that location here.]

VMS configuration system has been rewritten. See README.vms.

OpenBSD better supported. [XXX what others?]

Modules and Pragmata

New Modules


Perl compiler and tools. See [XXX what?].


A module to pretty print Perl data. See Data::Dumper.


A module to look up errors more conveniently. See Errno.


A portable API for file operations.


Query and manage installed modules.


Manipulate .packlist files.


Make functions/builtins succeed or die.


Constants and other support infrastructure for System V IPC operations in perl.


A framework for writing testsuites.


Base class for tied arrays.


Base class for tied handles.


Perl thread creation, manipulation, and support.


Set subroutine attributes.


Compile-time class fields.


Various pragmata to control behavior of regular expressions.

Changes in existing modules


CGI has been updated to version 2.42.


POSIX now has its own platform-specific hints files.


DB_File supports version 2.x of Berkeley DB. See ext/DB_File/Changes.


MakeMaker now supports writing empty makefiles, provides a way to specify that site umask() policy should be honored. There is also better support for manipulation of .packlist files, and getting information about installed modules.

Extensions that have both architecture-dependent and architecture-independent files are now always installed completely in the architecture-dependent locations. Previously, the shareable parts were shared both across architectures and across perl versions and were therefore liable to be overwritten with newer versions that might have subtle incompatibilities.


[XXX What?]


Cwd::cwd is faster on most platforms.


Keeps better time.

Utility Changes

h2ph and related utilities have been vastly overhauled.

perlcc, a new experimental front end for the compiler is available.

The crude GNU configure emulator is now called configure.gnu.

API Changes

Incompatible Changes

Deprecations, Extensions

C++ Support

Documentation Changes

Config.pm now has a glossary of variables.

Porting/patching.pod has detailed instructions on how to create and submit patches for perl.

New Diagnostics

Ambiguous call resolved as CORE::%s(), qualify as such or use &

(W) A subroutine you have declared has the same name as a Perl keyword, and you have used the name without qualification for calling one or the other. Perl decided to call the builtin because the subroutine is not imported.

To force interpretation as a subroutine call, either put an ampersand before the subroutine name, or qualify the name with its package. Alternatively, you can import the subroutine (or pretend that it's imported with the use subs pragma).

To silently interpret it as the Perl operator, use the CORE:: prefix on the operator (e.g. CORE::log($x)) or by declaring the subroutine to be an object method (see attrs).

Bad index while coercing array into hash

(F) The index looked up in the hash found as the 0'th element of a pseudo-hash is not legal. Index values must be at 1 or greater. See perlref.

Bareword "%s" refers to nonexistent package

(W) You used a qualified bareword of the form Foo::, but the compiler saw no other uses of that namespace before that point. Perhaps you need to predeclare a package?

Can't call method "%s" on an undefined value

(F) You used the syntax of a method call, but the slot filled by the object reference or package name contains an undefined value. Something like this will reproduce the error:

    $BADREF = 42;
    process $BADREF 1,2,3;
Can't coerce array into hash

(F) You used an array where a hash was expected, but the array has no information on how to map from keys to array indices. You can do that only with arrays that have a hash reference at index 0.

Can't goto subroutine from an eval-string

(F) The "goto subroutine" call can't be used to jump out of an eval "string". (You can use it to jump out of an eval {BLOCK}, but you probably don't want to.)

Can't use %%! because Errno.pm is not available

(F) The first time the %! hash is used, perl automatically loads the Errno.pm module. The Errno module is expected to tie the %! hash to provide symbolic names for $! errno values.

Can't use %%! because Errno.pm is not available

(F) The first time the %! hash is used, perl automatically loads the Errno.pm module. The Errno module is expected to tie the %! hash to provide symbolic names for $! errno values.

Cannot find an opnumber for "%s"

(F) A string of a form CORE::word was given to prototype(), but there is no builtin with the name word.

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: "\[." and ".\]".

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: "\[:" and ":\]".

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: "\[=" and "=\]".

%s: Eval-group in insecure regular expression

(F) Perl detected tainted data when trying to compile a regular expression that contains the (?{ ... }) zero-width assertion, which is unsafe. See "(?{ code })" in perlre, and perlsec.

%s: Eval-group not allowed, use re 'eval'

(F) A regular expression contained the (?{ ... }) zero-width assertion, but that construct is only allowed when the use re 'eval' pragma is in effect. See "(?{ code })" in perlre.

%s: Eval-group not allowed at run time

(F) Perl tried to compile a regular expression containing the (?{ ... }) zero-width assertion at run time, as it would when the pattern contains interpolated values. Since that is a security risk, it is not allowed. If you insist, you may still do this by explicitly building the pattern from an interpolated string at run time and using that in an eval(). See "(?{ code })" in perlre.

Explicit blessing to '' (assuming package main)

(W) You are blessing a reference to a zero length string. This has the effect of blessing the reference into the package main. This is usually not what you want. Consider providing a default target package, e.g. bless($ref, $p or 'MyPackage');

Illegal hex digit ignored

(W) You may have tried to use a character other than 0 - 9 or A - F in a hexadecimal number. Interpretation of the hexadecimal number stopped before the illegal character.

No such array field

(F) You tried to access an array as a hash, but the field name used is not defined. The hash at index 0 should map all valid field names to array indices for that to work.

No such field "%s" in variable %s of type %s

(F) You tried to access a field of a typed variable where the type does not know about the field name. The field names are looked up in the %FIELDS hash in the type package at compile time. The %FIELDS hash is usually set up with the 'fields' pragma.

Out of memory during ridiculously large request

(F) You can't allocate more than 2^31+"small amount" bytes. This error is most likely to be caused by a typo in the Perl program. e.g., $arr[time] instead of $arr[$time].

Range iterator outside integer range

(F) One (or both) of the numeric arguments to the range operator ".." are outside the range which can be represented by integers internally. One possible workaround is to force Perl to use magical string increment by prepending "0" to your numbers.

Recursive inheritance detected while looking for method '%s' in package '%s'

(F) More than 100 levels of inheritance were encountered while invoking a method. Probably indicates an unintended loop in your inheritance hierarchy.

Reference found where even-sized list expected

(W) You gave a single reference where Perl was expecting a list with an even number of elements (for assignment to a hash). This usually means that you used the anon hash constructor when you meant to use parens. In any case, a hash requires key/value pairs.

    %hash = { one => 1, two => 2, };   # WRONG
    %hash = [ qw/ an anon array / ];   # WRONG
    %hash = ( one => 1, two => 2, );   # right
    %hash = qw( one 1 two 2 );                 # also fine
Undefined value assigned to typeglob

(W) An undefined value was assigned to a typeglob, a la *foo = undef. This does nothing. It's possible that you really mean undef *foo.

Use of reserved word "%s" is deprecated

(D) The indicated bareword is a reserved word. Future versions of perl may use it as a keyword, so you're better off either explicitly quoting the word in a manner appropriate for its context of use, or using a different name altogether. The warning can be suppressed for subroutine names by either adding a & prefix, or using a package qualifier, e.g. &our(), or Foo::our().

perl: warning: Setting locale failed.

(S) The whole warning message will look something like:

       perl: warning: Setting locale failed.
       perl: warning: Please check that your locale settings:
               LC_ALL = "En_US",
               LANG = (unset)
           are supported and installed on your system.
       perl: warning: Falling back to the standard locale ("C").

Exactly what were the failed locale settings varies. In the above the settings were that the LC_ALL was "En_US" and the LANG had no value. This error means that Perl detected that you and/or your system administrator have set up the so-called variable system but Perl could not use those settings. This was not dead serious, fortunately: there is a "default locale" called "C" that Perl can and will use, the script will be run. Before you really fix the problem, however, you will get the same error message each time you run Perl. How to really fix the problem can be found in perllocale section LOCALE PROBLEMS.

Obsolete Diagnostics

Can't mktemp()

(F) The mktemp() routine failed for some reason while trying to process a -e switch. Maybe your /tmp partition is full, or clobbered.

Can't write to temp file for -e: %s

(F) The write routine failed for some reason while trying to process a -e switch. Maybe your /tmp partition is full, or clobbered.

Cannot open temporary file

(F) The create routine failed for some reason while trying to process a -e switch. Maybe your /tmp partition is full, or clobbered.


If you find what you think is a bug, you might check the headers of recently posted articles in 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 you 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.


2 POD Errors

The following errors were encountered while parsing the POD:

Around line 51:

'=item' outside of any '=over'

Around line 55:

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