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perldelta - what is new for perl v5.15.6


This document describes differences between the 5.15.5 release and the 5.15.6 release.

If you are upgrading from an earlier release such as 5.15.4, first read perl5155delta, which describes differences between 5.15.4 and 5.15.5.

Core Enhancements


The new __SUB__ token, available under the "current_sub" feature (see feature) or use v5.15, returns a reference to the current subroutine, making it easier to write recursive closures.

New option for the debugger's t command

The t command in the debugger, which toggles tracing mode, now accepts a numeric argument that determines how many levels of subroutine calls to trace.

Return value of tied

The value returned by tied on a tied variable is now the actual scalar that holds the object to which the variable is tied. This allows ties to be weakened with Scalar::Util::weaken(tied $tied_variable).



The XS-callable function is_utf8_char(), when presented with malformed UTF-8 input, can read up to 12 bytes beyond the end of the string. This cannot be fixed without changing its API. It is not called from CPAN. The documentation now describes how to use it safely.

Other is_utf8_foo() functions, as well as utf8_to_foo(), etc.

Most of the other XS-callable functions that take UTF-8 encoded input implicitly assume that the UTF-8 is valid (not malformed) in regards to buffer length. Do not do things such as change a character's case or see if it is alphanumeric without first being sure that it is valid UTF-8. This can be safely done for a whole string by using one of the functions is_utf8_string(), is_utf8_string_loc(), and is_utf8_string_loclen().


As of this release, version declarations like use v5.16 now disable all features before enabling the new feature bundle. This means that the following holds true:

    use 5.016;
    # 5.16 features enabled here
    use 5.014;
    # 5.16 features disabled here

use v5.12 and higher continue to enable strict, but explicit use strict and no strict now override the version declaration, even when they come first:

    no strict;
    use 5.012;
    # no strict here

There is a new ":default" feature bundle that represents the set of features enabled before any version declaration or use feature has been seen. Version declarations below 5.10 now enable the ":default" feature set. This does not actually change the behaviour of use v5.8, because features added to the ":default" set are those that were traditionally enabled by default, before they could be turned off.

$[ is now disabled under use v5.16. It is part of the default feature set and can be turned on or off explicitly with use feature 'array_base'.


The change to UNIVERSAL::VERSION in 5.15.2 has been reverted. It now returns a stringified version object once more.

substr lvalue revamp

When substr is called in lvalue or potential lvalue context with two or three arguments, a special lvalue scalar is returned that modifies the original string (the first argument) when assigned to.

Previously, the offsets (the second and third arguments) passed to substr would be converted immediately to match the string, negative offsets being translated to positive and offsets beyond the end of the string being truncated.

Now, the offsets are recorded without modification in the special lvalue scalar that is returned, and the original string is not even looked at by substr itself, but only when the returned lvalue is read or modified.

These changes result in several incompatible changes and bug fixes:

  • If the original string changes length after the call to substr but before assignment to its return value, negative offsets will remember their position from the end of the string, affecting code like this:

        my $string = "string";
        my $lvalue = \substr $string, -4, 2;
        print $lvalue, "\n"; # prints "ri"
        $string = "bailing twine";
        print $lvalue, "\n"; # prints "wi"; used to print "il"

    The same thing happens with an omitted third argument. The returned lvalue will always extend to the end of the string, even if the string becomes longer.

  • Tied (and otherwise magical) variables are no longer exempt from the "Attempt to use reference as lvalue in substr" warning.

  • That warning now occurs when the returned lvalue is assigned to, not when substr itself is called. This only makes a difference if the return value of substr is referenced and assigned to later.

  • The order in which "uninitialized" warnings occur for arguments to substr has changed.

  • Passing a substring of a read-only value or a typeglob to a function (potential lvalue context) no longer causes an immediate "Can't coerce" or "Modification of a read-only value" error. That error only occurs if and when the value passed is assigned to.

    The same thing happens with the "substr outside of string" error. If the lvalue is only read, not written to, it is now just a warning, as with rvalue substr.

  • substr assignments no longer call FETCH twice if the first argument is a tied variable, just once.

It was impossible to fix all the bugs without an incompatible change, and the behaviour of negative offsets was never specified, so the change was deemed acceptable.

Return value of eval

eval returns undef in scalar context or an empty list in list context when there is a run-time error. When eval was passed a string in list context and a syntax error occurred, it used to return a list containing a single undefined element. Now it returns an empty list in list context for all errors [perl #80630].

Anonymous handles

Automatically generated file handles are now named __ANONIO__ when the variable name cannot be determined, rather than $__ANONIO__.

Last-accessed filehandle

Perl has an internal variable that stores the last filehandle to be accessed. It is used by $. and by tell and eof without arguments.

It used to be possible to set this internal variable to a glob copy and then modify that glob copy to be something other than a glob, and still have the last-accessed filehandle associated with the variable after assigning a glob to it again:

    my $foo = *STDOUT;  # $foo is a glob copy
    <$foo>;             # $foo is now the last-accessed handle
    $foo = 3;           # no longer a glob
    $foo = *STDERR;     # still the last-accessed handle

Now the $foo = 3 assignment unsets that internal variable, so there is no last-accessed filehandle, just as if <$foo> had never happened.

XS API tweak

The newCONSTSUB_flags C-level function, added in 5.15.4, now has a len parameter.

Performance Enhancements

  • Perl 5.12.0 sped up the destruction of objects whose classes define empty DESTROY methods (to prevent autoloading), by simply not calling such empty methods. This release takes this optimisation a step further, by not calling any DESTROY method that begins with a return statement. This can be useful for destructors that are only used for debugging:

        use constant DEBUG => 1;
        sub DESTROY { return unless DEBUG; ... }

    Constant-folding will reduce the first statement to return; if DEBUG is set to 0, triggering this optimisation.

  • Assigning to a variable that holds a typeglob or copy-on-write scalar is now much faster. Previously the typeglob would be stringified or the copy-on-write scalar would be copied before being clobbered.

  • Assignment to substr in void context is now more than twice its previous speed. Instead of creating and returning a special lvalue scalar that is then assigned to, substr modifies the original string itself.

Modules and Pragmata

Updated Modules and Pragmata

  • Archive::Tar has been upgraded from version 1.80 to version 1.82.

    Adjustments to handle files >8gb (>0777777777777 octal) and a feature to return the MD5SUM of files in the archive.

  • AutoLoader has been upgraded from version 5.71 to version 5.72.

  • B::Debug has been upgraded from version 1.16 to version 1.17.

  • B::Deparse has been upgraded from version 1.09 to version 1.10.

    Various constructs that used to be deparsed incorrectly have been fixed:


    It used to deparse as sort foo(bar), which makes foo the sort routine, rather than a regular function call.

    Keys and values in %^H

    Undefined values in the hint hash were being deparsed as empty strings. Whenever the hint hash changed, all undefined values, even those unmodified, were being printed.

    Special characters, such as quotation marks, were not being escaped properly.

    Some values used to be omitted if, for instance, a key was the same as a previous value and vice versa.

    "method BLOCK" syntax

    method { $expr } used to be deparsed as something like do{ $expr }->method, but the latter puts the $expr in scalar context, whereas the former puts in list context.

    do +{} and do({})

    These are both variants of do-file syntax, but were being deparsed as do-blocks.

    Keywords that do not follow the llaf rule

    Keywords like return and last that do not follow the looks-like-a-function rule are now deparsed correctly with parentheses in the right place.

    Similarly, not, which does follow the llaf rule, was being deparsed as though it does not.


    In various cases, B::Deparse started adding a spurious $_ =~ before the right-hand side in Perl 5.14; e.g., "" =~ <$a> would become "" =~ ($_ =~ <$a>).

    open local *FH

    open, pipe and other functions that autovivify handles used to omit local * from local *FH.

    Negated single-letter subroutine calls

    Negated subroutine calls like - f() and -(f()) were being deparsed as file test operators.


    &{&} and & &, which are calls to the subroutine named "&", believe it or not, were being deparsed as &&.

    require $this + $that

    In Perl 5.14, require followed by any binary operator started deparsing as no.

  • Carp has been upgraded from version 1.23 to version 1.24.

    It now tacks the last-accessed filehandle and line number on to the end of the error message, just like die [perl #96672].

  • Compress::Raw::Zlib has been upgraded from version 2.042 to version 2.045.

  • Compress::Raw::Bzip2 has been upgraded from version 2.042 to version 2.045.

  • CPAN::Meta::YAML has been upgraded from version 0.004 to version 0.005.

  • CPANPLUS has been upgraded from version 0.9112 to version 0.9113.

  • Data::Dumper has been upgraded from version 2.134 to version 2.135.

    The XS implementation has been updated to account for the Unicode symbol changes in Perl 5.15.4. It also knows how to output typeglobs with nulls in their names.

  • diagnostics has been upgraded from version 1.25 to version 1.26.

    It now understands the "%X" format code, which some error messages started using in Perl 5.14.0.

  • Digest::SHA has been upgraded from version 5.63 to version 5.70.

    Added BITS mode to the addfile method and shasum. This makes partial-byte inputs possible via files/STDIN and allows shasum to check all 8074 NIST Msg vectors, where previously special programming was required to do this.

  • Exporter has been upgraded from version 5.65 to version 5.66.

    It no longer tries to localise $_ unnecessarily.

  • ExtUtils::ParseXS has been upgraded from version 3.05 to version 3.07.

  • IO::Compress::Base has been upgraded from version 2.042 to version 2.046.

    Added zipdetails utility.

  • Locale::Codes has been upgraded from version 3.18 to version 3.20.

    The code2XXX, XXX2code, all_XXX_codes, and all_XXX_names functions now support retired codes. All codesets may be specified by a constant or by their name now. Previously, they were specified only by a constant.

    The alias_code function exists for backward compatibility. It has been replaced by rename_country_code. The alias_code function will be removed some time after September, 2013.

    All work is now done in the central module (Locale::Codes). Previously, some was still done in the wrapper modules (Locale::Codes::*). Added Language Family codes (langfam) as defined in ISO 639-5.

  • Module::CoreList has been upgraded from version 2.58 to version 2.59.

  • Module::Loaded has been upgraded from version 0.06 to version 0.08.

  • Pod::LaTeX has been upgraded from version 0.59 to version 0.60.

    Added another LaTeX escape: --- => -{}-{}-

    Pod::LaTeX doesn't handle -- in PODs specially, passing it directly to LaTeX, which then proceeds to replace it with a single -. This patch replaces ----- with -{}-{}-{}-{}-

  • POSIX has been upgraded from version 1.26 to version 1.27.

    It no longer produces a "Constant subroutine TCSANOW redefined" warning on Windows.

    This bug was introduced in Perl 5.15.3.

  • Socket has been upgraded from version 1.94_02 to version 1.97.

  • threads has been upgraded from version 1.85 to version 1.86.

  • Unicode::Collate has been upgraded from version 0.85 to version 0.87.

    Tailored compatibility ideographs as well as unified ideographs for the locales: ja, ko, zh__big5han, zh__gb2312han, zh__pinyin, zh__stroke.

    Locale/*.pl files are now searched for in @INC.

  • UNIVERSAL has been upgraded from version 1.10 to version 1.11.

    Documentation change clarifies return values from UNIVERSAL::VERSION.

Removed Modules and Pragmata

  • Changing the case of a UTF-8 encoded string under use locale now gives better, but still imperfect, results. Previously, such a string would entirely lose locale semantics and silently be treated as Unicode. Now, the code points that are less than 256 are treated with locale rules, while those above 255 are, of course, treated as Unicode. See "lc" in perlfunc for more details, including the deficiencies of this scheme.


Changes to Existing Documentation

"Laundering and Detecting Tainted Data" in perlsec

  • The example function for checking for taintedness contained a subtle error. $@ needs to be localized to prevent its changing this global's value outside the function. The preferred method to check for this remains "tainted" in Scalar::Util.


Changes to Existing Diagnostics

  • Redefinition warnings for constant subroutines used to be mandatory, even occurring under no warnings. Now they respect the warnings pragma.

  • The "Attempt to free non-existent shared string" has had the spelling of "non-existent" corrected to "nonexistent". It was already listed with the correct spelling in perldiag.

  • The 'Use of "foo" without parentheses is ambiguous' warning has been extended to apply also to user-defined subroutines with a (;$) prototype, and not just to built-in functions.

  • The error messages for using default and when outside of a topicalizer have been standardised to match the messages for continue and loop controls. They now read 'Can't "default" outside a topicalizer' and 'Can't "when" outside a topicalizer'. They both used to be 'Can't use when() outside a topicalizer' [perl #91514].

Utility Changes


  • zipdetails displays information about the internal record structure of the zip file. It is not concerned with displaying any details of the compressed data stored in the zip file.

Configuration and Compilation

  • pod/roffitall is now built by pod/buildtoc, instead of being shipped with the distribution. Its list of manpages is now generated (and therefore current). See also RT #103202 for an unresolved related issue.

  • Perl 5.15.5 had a bug in its installation script, which did not install unicore/Name.pm. This has been corrected [perl #104226].

  • The man page for XS::Typemap is no longer installed. XS::Typemap is a test module which is not installed, hence installing its documentation makes no sense.

  • The man pages for the perl FAQ, perlxs, perlxstut and perldoc are once again correctly installed in man1, not man3

  • The -Dusesitecustomize and -Duserelocatableinc options now work together properly.


  • The substr.t and substr_thr.t scripts for testing substr have been moved under t/op/, where they were originally. They had been moved under t/re/ along with the substitution tests when that directory was created.

Platform Support

Platform-Specific Notes


  • A link-time error on VMS versions without symlink support was introduced in 5.15.1, but has now been corrected.

  • Explicit support for VMS versions prior to v7.0 and DEC C versions prior to v6.0 has been removed.

  • Since Perl 5.10.1, the home-grown stat wrapper has been unable to distinguish between a directory name containing an underscore and an otherwise-identical filename containing a dot in the same position (e.g., t/test_pl as a directory and t/test.pl as a file). This problem has been corrected.

Selected Bug Fixes

  • RT #78266: The regex engine has been leaking memory when accessing named captures that weren't matched as part of a regex ever since 5.10 when they were introduced, e.g. this would consume over a hundred MB of memory:

        for (1..10_000_000) {
            if ("foo" =~ /(foo|(?<capture>bar))?/) {
                my $capture = $+{capture}
        system "ps -o rss $$"'
  • A constant subroutine assigned to a glob whose name contains a null will no longer cause extra globs to pop into existence when the constant is referenced under its new name.

  • sort was not treating sub {} and sub {()} as equivalent when such a sub was provided as the comparison routine. It used to croak on sub {()}.

  • Subroutines from the autouse namespace are once more exempt from redefinition warnings. This used to work in 5.005, but was broken in 5.6 for most subroutines. For subs created via XS that redefine subroutines from the autouse package, this stopped working in 5.10.

  • New XSUBs now produce redefinition warnings if they overwrite existing subs, as they did in 5.8.x. (The autouse logic was reversed in 5.10-14. Only subroutines from the autouse namespace would warn when clobbered.)

  • Redefinition warnings triggered by the creation of XSUBs now respect Unicode glob names, instead of using the internal representation. This was missed in 5.15.4, partly because this warning was so hard to trigger. (See the previous item.)

  • newCONSTSUB used to use compile-time warning hints, instead of run-time hints. The following code should never produce a redefinition warning, but it used to, if newCONSTSUB redefined an existing subroutine:

        use warnings;
        BEGIN {
            no warnings;
  • Redefinition warnings for constant subroutines are on by default (what are known as severe warnings in perldiag). This was only the case when it was a glob assignment or declaration of a Perl subroutine that caused the warning. If the creation of XSUBs triggered the warning, it was not a default warning. This has been corrected.

  • The internal check to see whether a redefinition warning should occur used to emit "uninitialized" warnings in cases like this:

        use warnings "uninitialized";
        use constant {u => undef, v => undef};
        sub foo(){u}
        sub foo(){v}
  • A bug fix in Perl 5.14 introduced a new bug, causing "uninitialized" warnings to report the wrong variable if the operator in question had two operands and one was %{...} or @{...}. This has been fixed [perl #103766].

  • version->new("version") and printf "%vd", "version" no longer crash [perl #102586].

  • $tied =~ y/a/b/, chop $tied and chomp $tied now call FETCH just once when $tied holds a reference.

  • Four-argument select now always calls FETCH on tied arguments. It used to skip the call if the tied argument happened to hold undef or a typeglob.

  • Four-argument select no longer produces its "Non-string passed as bitmask" warning on tied or tainted variables that are strings.

  • sysread now always calls FETCH on the buffer passed to it if the buffer is tied. It used to skip the call if the tied variable happened to hold a typeglob.

  • $tied .= <> now calls FETCH once on $tied. It used to call it multiple times if the last value assigned to or returned from the tied variable was anything other than a string or typeglob.

  • The evalbytes keyword added in 5.15.5 was respecting use utf8 declarations from the outer scope, when it should have been ignoring them.

  • goto &func no longer crashes, but produces an error message, when the unwinding of the current subroutine's scope fires a destructor that undefines the subroutine being "goneto" [perl #99850].

  • Arithmetic assignment ($left += $right) involving overloaded objects that rely on the 'nomethod' override no longer segfault when the left operand is not overloaded.

  • Assigning __PACKAGE__ or any other shared hash key scalar to a stash element no longer causes a double free. Regardless of this change, the results of such assignments are still undefined.

  • Assigning __PACKAGE__ or another shared hash key string to a variable no longer stops that variable from being tied if it happens to be a PVMG or PVLV internally.

  • Creating a UNIVERSAL::AUTOLOAD sub no longer stops %+, %- and %! from working some of the time [perl #105024].

  • When presented with malformed UTF-8 input, the XS-callable functions is_utf8_string(), is_utf8_string_loc(), and is_utf8_string_loclen() could read beyond the end of the input string by up to 12 bytes. This no longer happens. [perl #32080]. However, currently, is_utf8_char() still has this defect, see "is_utf8_char()" above.

  • Doing a substitution on a tied variable returning a copy-on-write scalar used to cause an assertion failure or an "Attempt to free nonexistent shared string" warning.

  • A change in perl 5.15.4 caused caller() to produce malloc errors and a crash with Perl's own malloc, and possibly with other malloc implementations, too [perl #104034].

  • A bug fix in 5.15.5 could sometimes result in assertion failures under debugging builds of perl for certain syntax errors in eval, such as eval q|""!=!~//|

  • The "c [line num]" debugger command was broken by other debugger changes released in 5.15.3. This is now fixed.

  • Breakpoints were not properly restored after a debugger restart using the "R" command. This was broken in 5.15.3. This is now fixed.

  • The debugger prompt did not display the current line. This was broken in 5.15.3. This is now fixed.

  • Class method calls still suffered from the Unicode bug with Latin-1 package names. This was missed in the Unicode package name cleanup in 5.15.4 [perl #105922].

  • The debugger no longer tries to do local $_ when dumping data structures.

  • Calling readline($fh) where $fh is a glob copy (e.g., after $fh = *STDOUT), assigning something other than a glob to $fh, and then freeing $fh (e.g., by leaving the scope where it is defined) no longer causes the internal variable used by $. (PL_last_in_gv) to point to a freed scalar, that could be reused for some other glob, causing $. to use some unrelated filehandle [perl #97988].

  • A regression in 5.14 caused these statements not to set the internal variable that holds the handle used by $.:

        my $fh = *STDOUT;
        tell $fh;
        eof  $fh;
        seek $fh, 0,0;
        tell     *$fh;
        eof      *$fh;
        seek     *$fh, 0,0;
        readline *$fh;

    This is now fixed, but tell *{ *$fh } still has the problem, and it is not clear how to fix it [perl #106536].

  • Version comparisons, such as those that happen implicitly with use v5.43, no longer cause locale settings to change [perl #105784].

  • pod/buildtoc, which generates perltoc, put path names in the perltoc file. This bug was introduced in 5.15.1.


Perl 5.15.6 represents approximately 2 months of development since Perl 5.15.5 and contains approximately 48,000 lines of changes across 560 files from 36 authors.

Perl continues to flourish into its third decade thanks to a vibrant community of users and developers. The following people are known to have contributed the improvements that became Perl 5.15.6:

Aaron Crane, Abhijit Menon-Sen, Alexandr Ciornii, Brian Fraser, Carl Hayter, Chris 'BinGOs' Williams, Craig A. Berry, Dave Rolsky, David Golden, David Mitchell, Dominic Hargreaves, Father Chrysostomos, James E Keenan, Johannes Plunien, John Peacock, Karl Williamson, Marc Green, Mark Dootson, Matthew Horsfall, Nicholas Clark, Paul Evans, Peter Martini, Peter Scott, Rafael Garcia-Suarez, Reini Urban, Ricardo Signes, Rodolfo Carvalho, Shlomi Fish, Steffen Müller, Steve Hay, Steve Peters, Thomas Sibley, Timothe Litt, Tony Cook, Vadim Konovalov, Ævar Arnfjörð Bjarmason.

The list above is almost certainly incomplete as it is automatically generated from version control history. In particular, it does not include the names of the (very much appreciated) contributors who reported issues to the Perl bug tracker.

Many of the changes included in this version originated in the CPAN modules included in Perl's core. We're grateful to the entire CPAN community for helping Perl to flourish.

For a more complete list of all of Perl's historical contributors, please see the AUTHORS file in the Perl source distribution.

Reporting Bugs

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

If you believe you have an unreported bug, please run the perlbug program included with your release. Be 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.org to be analysed by the Perl porting team.

If the bug you are reporting has security implications, which make it inappropriate to send to a publicly archived mailing list, then please send it to perl5-security-report@perl.org. This points to a closed subscription unarchived mailing list, which includes all the core committers, who will be able to help assess the impact of issues, figure out a resolution, and help co-ordinate the release of patches to mitigate or fix the problem across all platforms on which Perl is supported. Please only use this address for security issues in the Perl core, not for modules independently distributed on CPAN.


The Changes file for an explanation of how to view 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.