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Andrew Main (Zefram)
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perldelta - what is new for perl v5.17.0


This document describes differences between the 5.16.0 release and the 5.17.0 release.

If you are upgrading from an earlier release such as 5.16.0, first read perl5160delta, which describes differences between 5.14.0 and 5.16.0.

Incompatible Changes

qw(...) can no longer be used as parentheses

qw lists used to fool the parser into thinking they were always surrounded by parentheses. This permitted some surprising constructions such as foreach $x qw(a b c) {...}, which should really be written foreach $x (qw(a b c)) {...}. These would sometimes get the lexer into the wrong state, so they didn't fully work, and the similar foreach qw(a b c) {...} that one might expect to be permitted never worked at all.

This side effect of qw has now been abolished. It has been deprecated since Perl 5.13.11. It is now necessary to use real parentheses everywhere that the grammar calls for them.

\s in regular expressions now matches a Vertical Tab (experimental)

This is an experiment early in the development cycle to see what repercussions arise from this change. It may well be that we decide to require a "use feature" to activate this behavior. Because of the experimental nature of this, which may be reversed, the documentation has not been changed to reflect it.


Unescaped braces in regexps

Literal unescaped { in regular expressions is now deprecated. Every brace character should be either escaped (by a preceding backslash) or part of a construct where it's a metacharacter. This catches likely typos such as /f{,3}/. It will also allow braces to be used in the future to delimit parameters to metacharacters that currently take no parameters.

Performance Enhancements

  • Filetest ops manage the stack in a fractionally more efficient manner.

  • Globs used in a numeric context are now numerified directly in most cases, rather than being numerified via stringification.

Modules and Pragmata

Updated Modules and Pragmata

  • B has been upgraded from version 1.34 to 1.35.

    The stashlen method of COPs has been added. This provides access to an internal field added in perl 5.16 under threaded builds [perl #113034].

  • ExtUtils::ParseXS has been upgraded from version 3.16 to 3.17.

    The generated C code now avoids unnecessarily incrementing PL_amagic_generation on Perl versions where it's done automatically (or on current Perl where the variable no longer exists).

  • File::DosGlob has been upgraded from version 1.06 to 1.07.

    Small documentation and comment fixes.

  • File::Find has been upgraded from version 1.20 to 1.21.

    Individual files may now appear in list of directories to be searched [perl #59750].

  • File::stat has been upgraded from version 1.05 to 1.06.

    File::stat ignores the filetest pragma, and warns when used in combination therewith. But it was not warning for -r. This has been fixed [perl #111640].

    -p now works, and does not return false for pipes [perl #111638].

  • Hash::Util has been upgraded from version 0.11 to 0.12.

    hash_unlocked and hashref_unlocked now returns true if the hash is unlocked, instead of always returning false [perl #112126].

    hash_unlocked, hashref_unlocked, lock_hash_recurse and unlock_hash_recurse are now exportable [perl #112126].

    Two new functions, hash_locked and hashref_locked, have been added. Oddly enough, these two functions were already exported, even though they did not exist [perl #112126].

  • Module::CoreList has been upgraded from version 2.66 to 2.67.

  • overload has been upgraded from version 1.18 to 1.19.

    no overload now warns for invalid arguments, just like use overload.

  • Pod::Functions has been upgraded from version 1.05 to 1.06.

    Typo fix in generated documentation.

  • Storable has been upgraded from version 2.34 to 2.35.

    Modifying $_[0] within STORABLE_freeze no longer results in crashes [perl #112358].

    An object whose class implements STORABLE_attach is now thawed only once when there are multiple references to it in the structure being thawed [perl #111918].

  • utf8 has been upgraded from version 1.09 to 1.10.

    Some documentation has been clarified.

Removed Modules and Pragmata


Changes to Existing Documentation


  • Now explicitly documents the behaviour of hash initializer lists that contain duplicate keys.


  • The explanation of symbolic references being prevented by "strict refs" now doesn't assume that the reader knows what symbolic references are.


  • The return value of pipe is now documented.


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.

New Diagnostics

New Warnings


  • The test suite now has a section for tests that require very large amounts of memory. These tests won't run by default; they can be enabled by setting the PERL_TEST_MEMORY environment variable to the number of gibibytes of memory that may be safely used.

Platform Support

Platform-Specific Notes


There is now a workaround for a compiler bug that prevented compiling with clang++ since Perl 5.15.7 [perl #112786].


When compiling the Perl core as C++ (which is only semi-supported), the mathom functions are now compiled as extern "C", to ensure proper binary compatibility. (However, binary compatibility isn't generally guaranteed anyway in the situations where this would matter.)


It should now be possible to compile Perl as C++ on VMS.

Internal Changes

  • The CV * typemap entry now supports &{} overloading and typeglobs, just like &{...} [perl #96872].

  • The SVf_AMAGIC flag to indicate overloading is now on the stash, not the object. It is now set automatically whenever a method or @ISA changes, so its meaning has changed, too. It now means "potentially overloaded". When the overload table is calculated, the flag is automatically turned off if there is no overloading, so there should be no noticeable slowdown.

    The staleness of the overload tables is now checked when overload methods are invoked, rather than during bless.

    "A" magic is gone. The changes to the handling of the SVf_AMAGIC flag eliminate the need for it.

    PL_amagic_generation has been removed as no longer necessary. For XS modules, it is now a macro alias to PL_na.

    The fallback overload setting is now stored in a stash entry separate from overloadedness itself.

  • The character-processing code has been cleaned up in places. The changes should be operationally invisible.

Selected Bug Fixes

  • goto '' now looks for an empty label, producing the "goto must have label" error message, instead of exiting the program [perl #111794].

  • goto "\0" now dies with "Can't find label" instead of "goto must have label".

  • The C function hv_store used to result in crashes when used on %^H [perl #111000].

  • A call checker attached to a closure prototype via cv_set_call_checker is now copied to closures cloned from it. So cv_set_call_checker now works inside an attribute handler for a closure.

  • Writing to $^N used to have no effect. Now it croaks with "Modification of a read-only value" by default, but that can be overridden by a custom regular expression engine, as with $1 [perl #112184].

  • undef on a control character glob (undef *^H) no longer emits an erroneous warning about ambiguity [perl #112456].

  • For efficiency's sake, many operators and built-in functions return the same scalar each time. Lvalue subroutines and subroutines in the CORE:: namespace were allowing this implementation detail to leak through. print &CORE::uc("a"), &CORE::uc("b") used to print "BB". The same thing would happen with an lvalue subroutine returning the return value of uc. Now the value is copied in such cases.

  • method {} syntax with an empty block or a block returning an empty list used to crash or use some random value left on the stack as its invocant. Now it produces an error.

  • vec now works with extremely large offsets (>2 GB) [perl #111730].

  • Changes to overload settings now take effect immediately, as do changes to inheritance that affect overloading. They used to take effect only after bless.

    Objects that were created before a class had any overloading used to remain non-overloaded even if the class gained overloading through use overload or @ISA changes, and even after bless. This has been fixed [perl #112708].

  • Classes with overloading can now inherit fallback values.

  • Overloading was not respecting a fallback value of 0 if there were overloaded objects on both sides of an assignment operator like += [perl #111856].

  • pos now croaks with hash and array arguments, instead of producing erroneous warnings.

  • while(each %h) now implies while(defined($_ = each %h)), like readline and readdir.

  • Subs in the CORE:: namespace no longer crash after undef *_ when called with no argument list (&CORE::time with no parentheses).

  • Unicode 6.1 published an incorrect alias for one of the Canonical_Combining_Class property's values (which range between 0 and 254). The alias CCC133 should have been CCC132. Perl now overrides the data file furnished by Unicode to give the correct value.

  • unpack no longer produces the "'/' must follow a numeric type in unpack" error when it is the data that are at fault [perl #60204].

  • join and "@array" now call FETCH only once on a tied $" [perl #8931].

  • Some subroutine calls generated by compiling core ops affected by a CORE::GLOBAL override had op checking performed twice. The checking is always idempotent for pure Perl code, but the double checking can matter when custom call checkers are involved.

  • A race condition used to exist around fork that could cause a signal sent to the parent to be handled by both parent and child. Signals are now blocked briefly around fork to prevent this from happening [perl #82580].


Perl 5.17.0 represents approximately 1 week of development since Perl 5.16.0 and contains approximately 17,000 lines of changes across 180 files from 26 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 5.16.1:

Alan Haggai Alavi, Aristotle Pagaltzis, Brian Fraser, Chris 'BinGOs' Williams, Craig A. Berry, Dagfinn Ilmari Mannsåker, Eric Brine, Father Chrysostomos, Igor Zaytsev, James E Keenan, Karl Williamson, Leon Timmermans, Lukas Mai, Marcus Holland-Moritz, Matthew Horsfall, Moritz Lenz, Rafael Garcia-Suarez, Reini Urban, Ricardo Signes, Robin Barker, Ronald J. Kimball, Smylers, Steffen Müller, Steve Hay, Tony Cook, Zefram.

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.