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[ this is a template for a new perldelta file. Any text flagged as XXX needs to be processed before release. ]

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.


XXX Any important notices here

Core Enhancements

XXX New core language features go here. Summarise user-visible core language enhancements. Particularly prominent performance optimisations could go here, but most should go in the "Performance Enhancements" section.

[ List each enhancement as a =head2 entry ]


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 numerical 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).


XXX Any security-related notices go here. In particular, any security vulnerabilities closed should be noted here rather than in the "Selected Bug Fixes" section.

[ List each security issue as a =head2 entry ]

Incompatible Changes

XXX For a release on a stable branch, this section aspires to be:

    There are no changes intentionally incompatible with 5.XXX.XXX
    If any exist, they are bugs and reports are welcome.

[ List each incompatible change as a =head2 entry ]

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 ot 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, but 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.

XS API tweak

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


XXX Any deprecated features, syntax, modules etc. should be listed here. In particular, deprecated modules should be listed here even if they are listed as an updated module in the "Modules and Pragmata" section.

[ List each deprecation as a =head2 entry ]

Performance Enhancements

XXX Changes which enhance performance without changing behaviour go here. There may well be none in a stable release.

[ List each enhancement as a =item entry ]

  • Perl 5.12.0 sped up the destruction of objects whose classes define empty DESTROY methods (to prevent autoloading), simply by not calling such empty methods. This release takes this optimisation a step further, by not calling any DESTROY method that begins with an 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.

  • Assign 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 a substring 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

XXX All changes to installed files in cpan/, dist/, ext/ and lib/ go here. If Module::CoreList is updated, generate an initial draft of the following sections using Porting/, which prints stub entries to STDOUT. Results can be pasted in place of the '=head2' entries below. A paragraph summary for important changes should then be added by hand. In an ideal world, dual-life modules would have a Changes file that could be cribbed.

[ Within each section, list entries as a =item entry ]

New Modules and Pragmata

  • XXX

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 1.10.

    sort(foo(bar)) is now deparsed correctly. (sort foo(bar), how it used to deparse, makes foo the sort routine, rather than a regular function call.)

  • 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.
  • CPANPLUS has been upgraded from version 0.9112 to version 0.9113.
  • Data::Dumper has been upgraded from version 2.134 to 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.

  • 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 2.045.

    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 sometime 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::*) but that is gone now. Added Language Family codes (langfam) as defined in ISO 639-5.

  • Module::Loaded has been uprgaded 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 1.27.

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

    XXX When did it start producing that warning? Was it post-5.15.5? Even if it was not, adding a note will help whoever compiles perl5160delta.

  • Socket has been upgraded from version 1.94_02 to version 1.96.
  • 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.

    Now Locale/*.pl files are searched in @INC.

  • UNIVERSAL has been upgraded from version 1.10 to 1.11.

    Documentation change clarifies return values from UNIVERSAL::VERSION.

Removed Modules and Pragmata

  • XXX


XXX Changes to files in pod/ go here. Consider grouping entries by file and be sure to link to the appropriate page, e.g. perlfunc.

New Documentation

XXX Changes which create new files in pod/ go here.


XXX Description of the purpose of the new file here

Changes to Existing Documentation

XXX Changes which significantly change existing files in pod/ go here. However, any changes to pod/perldiag.pod should go in the "Diagnostics" section.


  • XXX Description of the change here


The following additions or changes have been made to diagnostic output, including warnings and fatal error messages. For the complete list of diagnostic messages, see perldiag.

XXX New or changed warnings emitted by the core's C code go here. Also include any changes in perldiag that reconcile it to the C code.

[ Within each section, list entries as a =item entry that links to perldiag, e.g.

  =item *

  L<Invalid version object|perldiag/"Invalid version object">

New Diagnostics

XXX Newly added diagnostic messages go here

New Errors

New Warnings

Changes to Existing Diagnostics

XXX Changes (i.e. rewording) of diagnostic messages go here

  • 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.

Utility Changes

XXX Changes to installed programs such as perlbug and xsubpp go here. Most of these are built within the directories utils and x2p.

[ List utility changes as a =head3 entry for each utility and =item entries for each change Use XXX with program names to get proper documentation linking. ]


  • 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

XXX Changes to Configure, installperl, installman, and analogous tools go here. Any other changes to the Perl build process should be listed here. However, any platform-specific changes should be listed in the "Platform Support" section, instead.

[ List changes as a =item entry ].

  • pod/roffitall is now build 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/ This has been corrected [perl #104226].

    XXX Is that Perl version correct? Is the file path correct?


XXX Any significant changes to the testing of a freshly built perl should be listed here. Changes which create new files in t/ go here as do any large changes to the testing harness (e.g. when parallel testing was added). Changes to existing files in t/ aren't worth summarising, although the bugs that they represent may be covered elsewhere.

[ List each test improvement as a =item entry ]

  • 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

XXX Any changes to platform support should be listed in the sections below.

[ Within the sections, list each platform as a =item entry with specific changes as paragraphs below it. ]

New Platforms

XXX List any platforms that this version of perl compiles on, that previous versions did not. These will either be enabled by new files in the hints/ directories, or new subdirectories and README files at the top level of the source tree.



Discontinued Platforms

XXX List any platforms that this version of perl no longer compiles on.



Platform-Specific Notes

XXX List any changes for specific platforms. This could include configuration and compilation changes or changes in portability/compatibility. However, changes within modules for platforms should generally be listed in the "Modules and Pragmata" section.



Internal Changes

XXX Changes which affect the interface available to XS code go here. Other significant internal changes for future core maintainers should be noted as well.

[ List each change as a =item entry ]

  • XXX

Selected Bug Fixes

XXX Important bug fixes in the core language are summarised here. Bug fixes in files in ext/ and lib/ are best summarised in "Modules and Pragmata".

[ List each fix as a =item entry ]

  • 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 redefine and 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 has two operands and one is %{...} 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 it is tied. It used to skip the call if the tied variable happened to hold a typeglob.
  • $tied .= <> now calls FETCH once on $tied. It used to call it multiple times if the last value assigned to or returned from the tied variable was anything other than a string or typeglob.
  • The evalbytes keyword added in 5.15.5 was respecting use utf8 declarations from the outer scope, when it should have been ignoring them.
  • goto &func no longers crashes, but produces an error message, when the unwinding of the current subroutine's scope fires a destructor that undefines the subroutine being "goneto" [perl #99850].
  • Arithmetic assignment ($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.
  • Creating a UNIVERSAL::AUTOLOAD sub no longer stops %+, %- and %! from working some of the time [perl #105024].

Known Problems

XXX Descriptions of platform agnostic bugs we know we can't fix go here. Any tests that had to be TODOed for the release would be noted here, unless they were specific to a particular platform (see below).

This is a list of some significant unfixed bugs, which are regressions from either 5.XXX.XXX or 5.XXX.XXX.

[ List each fix as a =item entry ]

  • XXX


XXX If any significant core contributor has died, we've added a short obituary here.


XXX Generate this with:

  perl Porting/ v5.15.5..HEAD

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 . There may also be information at , 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 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 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.

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