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perldelta - what is new for perl v5.17.7


This document describes differences between the 5.17.6 release and the 5.17.7 release.

If you are upgrading from an earlier release such as 5.17.5, first read perl5176delta, which describes differences between 5.17.5 and 5.17.6.

Core Enhancements

$&, $` and $' are no longer slow

These three infamous variables have been redeemed and no longer slow down your program when used. Hence, the /p regular expression flag now does nothing.


Storable security warning in documentation

The documentation for Storable now includes a section which warns readers of the danger of accepting Storable documents from untrusted sources. The short version is that deserializing certain types of data can lead to loading modules and other code execution. This is documented behavior and wanted behavior, but this opens an attack vector for malicious entities.

Locale::Maketext allowed code injection via a malicious template

If users could provide a translation string to Locale::Maketext, this could be used to invoke arbitrary Perl subroutines available in the current process.

This has been fixed, but it is still possible to invoke any method provided by Locale::Maketext itself or a subclass that you are using. One of these methods in turn will invoke the Perl core's sprintf subroutine.

In summary, allowing users to provide translation strings without auditing them is a bad idea.

This vulnerability is documented in CVE-2012-6329.

Incompatible Changes

readline() with $/ = \N now reads N characters, not N bytes

Previously, when reading from a stream with I/O layers such as encoding, the readline() function, otherwise known as the <> operator, would read N bytes from the top-most layer. [perl #79960]

Now, N characters are read instead.

There is no change in behaviour when reading from streams with no extra layers, since bytes map exactly to characters.

Lexical subroutine warnings have moved

The warning about the use of an experimental feature emitted when lexical subroutines (added in 5.17.4) are used now happens when the subroutine itself is declared, not when the "lexical_subs" feature is activated via use feature.

This stops use feature ':all' from warning, but causes my sub foo; my sub bar to warn twice.

Overridden glob is now passed one argument

glob overrides used to be passed a magical undocumented second argument that identified the caller. Nothing on CPAN was using this, and it got in the way of a bug fix, so it was removed. If you really need to identify the caller, see Devel::Callsite on CPAN.


Lexical $_ is now deprecated

Since it was introduced in Perl 5.10, it has caused much confusion with no obvious solution:

  • Various modules (e.g., List::Util) expect callback routines to use the global $_. use List::Util 'first'; my $_; first { $_ == 1 } @list does not work as one would expect.

  • A my $_ declaration earlier in the same file can cause confusing closure warnings.

  • The "_" subroutine prototype character allows called subroutines to access your lexical $_, so it is not really private after all.

  • Nevertheless, subroutines with a "(@)" prototype and methods cannot access the caller's lexical $_, unless they are written in XS.

  • But even XS routines cannot access a lexical $_ declared, not in the calling subroutine, but in an outer scope, iff that subroutine happened not to mention $_ or use any operators that default to $_.

Various XS-callable functions are now deprecated

The following functions will be removed from a future version of Perl, and should not be used. With participating C compilers (e.g., gcc), compiling any file that uses any of these will generate a warning. These were not intended for public use; there are equivalent, faster, macros for most of them. See "Character classes" in perlapi: is_uni_ascii, is_uni_ascii_lc, is_uni_blank, is_uni_blank_lc, is_uni_cntrl, is_uni_cntrl_lc, is_uni_idfirst_lc, is_uni_space, is_uni_space_lc, is_uni_xdigit, is_uni_xdigit_lc, is_utf8_ascii, is_utf8_blank, is_utf8_cntrl, is_utf8_idcont, is_utf8_idfirst, is_utf8_perl_space, is_utf8_perl_word, is_utf8_posix_digit, is_utf8_space, is_utf8_xdigit. is_utf8_xidcont, is_utf8_xidfirst, to_uni_lower_lc, to_uni_title_lc, and to_uni_upper_lc.

Performance Enhancements

  • Perl has a new copy-on-write mechanism that avoids the need to copy the internal string buffer when assigning from one scalar to another. This makes copying large strings appear much faster. Modifying one of the two (or more) strings after an assignment will force a copy internally. This makes it unnecessary to pass strings by reference for efficiency.

Modules and Pragmata

Updated Modules and Pragmata

  • File::DosGlob has been upgraded from version 1.08 to 1.09. The internal cache of file names that it keeps for each caller is now freed when that caller is freed. This means use File::DosGlob 'glob'; eval 'scalar <*>' no longer leaks memory.

  • File::Glob has been upgraded from version 1.18 to 1.19. File::Glob has had exactly the same fix as File::DosGlob. Since it is what Perl's own glob operator itself uses (except on VMS), this means eval 'scalar <*>' no longer leaks.

  • GDBM_File has been upgraded from version 1.14 to 1.15. The undocumented optional fifth parameter to TIEHASH has been removed. This was intended to provide control of the callback used by gdbm* functions in case of fatal errors (such as filesystem problems), but did not work (and could never have worked). No code on CPAN even attempted to use it. The callback is now always the previous default, croak. Problems on some platforms with how the C croak function is called have also been resolved.

  • Module::CoreList has been upgraded from version 2.78 to 2.79.


Changes to Existing Documentation

"Character classes" in perlapi

  • There are quite a few macros callable from XS modules that classify characters into things like alphabetic, punctuation, etc. More of these are now documented, including ones which work on characters whose code points are outside the Latin-1 range.


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.

Changes to Existing Diagnostics

Internal Changes

  • SvUPGRADE() is no longer an expression. Originally this macro (and its underlying function, sv_upgrade()) were documented as boolean, although in reality they always croaked on error and never returned false. In 2005 the documentation was updated to specify a void return value, but SvUPGRADE() was left always returning 1 for backwards compatibility. This has now been removed, and SvUPGRADE() is now a statement with no return value.

    So this is now a syntax error:

        if (!SvUPGRADE(sv)) { croak(...); }

    If you have code like that, simply replace it with


    or to to avoid compiler warnings with older perls, possibly

  • Perl has a new copy-on-write mechanism that allows any SvPOK scalar to be upgraded to a copy-on-write scalar. A reference count on the string buffer is stored in the string buffer itself.

    This breaks a few XS modules by allowing copy-on-write scalars to go through code paths that never encountered them before.

    This behaviour can still be disabled by running Configure with -Accflags=-DPERL_NO_COW. This option will probably be removed in Perl 5.20.

  • Copy-on-write no longer uses the SvFAKE and SvREADONLY flags. Hence, SvREADONLY indicates a true read-only SV.

    Use the SvIsCOW macro (as before) to identify a copy-on-write scalar.

  • PL_sawampersand is now a constant. The switch this variable provided (to enable/disable the pre-match copy depending on whether $& had been seen) has been removed and replaced with copy-on-write, eliminating a few bugs.

    The previous behaviour can still be enabled by running Configure with -Accflags=-DPERL_SAWAMPERSAND.

  • PL_glob_index is gone.

Selected Bug Fixes

  • sort {undef} ... under fatal warnings no longer crashes. It started crashing in Perl 5.16.

  • Stashes blessed into each other (bless \%Foo::, 'Bar'; bless \%Bar::, 'Foo') no longer result in double frees. This bug started happening in Perl 5.16.

  • Numerous memory leaks have been fixed, mostly involving fatal warnings and syntax errors.

  • Lexical constants (my sub answer () { 42 }) no longer cause double frees.

  • Constant subroutine redefinition warns by default, but lexical constants were accidentally exempt from default warnings. This has been corrected.

  • Some failed regular expression matches such as 'f' =~ /../g were not resetting pos. Also, "match-once" patterns (m?...?g) failed to reset it, too, when invoked a second time [perl #23180].

  • Accessing $& after a pattern match now works if it had not been seen before the match. I.e., this applies to ${'&'} (under no strict) and eval '$&'. The same applies to $' and $` [perl #4289].

  • Several bugs involving local *ISA and local *Foo:: causing stale MRO caches have been fixed.

  • Defining a subroutine when its typeglob has been aliased no longer results in stale method caches. This bug was introduced in Perl 5.10.

  • Localising a typeglob containing a subroutine when the typeglob's package has been deleted from its parent stash no longer produces an error. This bug was introduced in Perl 5.14.

  • Under some circumstances, local *method=... would fail to reset method caches upon scope exit.

  • /[.foo.]/ is no longer an error, but produces a warning (as before) and is treated as /[.fo]/ [perl #115818].

  • goto $tied_var now calls FETCH before deciding what type of goto (subroutine or label) this is.

  • Renaming packages through glob assignment (*Foo:: = *Bar::; *Bar:: = *Baz::) in combination with m?...? and reset no longer makes threaded builds crash.

  • An earlier release in the 5.17.x series could crash if user code prevented _charnames from loading via $INC{'_charnames.pm'}++.

  • A number of bugs related to assigning a list to hash have been fixed. Many of these involve lists with repeated keys like (1, 1, 1, 1).


    The expression scalar(%h = (1, 1, 1, 1)) now returns 4, not 2.


    The return value of %h = (1, 1, 1) in list context was wrong. Previously this would return (1, undef, 1), now it returns (1, undef).


    Perl now issues the same warning on ($s, %h) = (1, {}) as it does for (%h) = ({}), "Reference found where even-sized list expected".


    A number of additional edge cases in list assignment to hashes were corrected. For more details see commit 23b7025ebc.

Known Problems

There may be a failure in the t/op/require_errors.t test if you run the test suite as the root user.


Perl 5.17.7 represents approximately 4 weeks of development since Perl 5.17.6 and contains approximately 30,000 lines of changes across 490 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 5.17.7:

Alexandr Ciornii, Bob Ernst, Brian Carlson, Chris 'BinGOs' Williams, Craig A. Berry, Daniel Dragan, Dave Rolsky, David Mitchell, Father Chrysostomos, Hugo van der Sanden, James E Keenan, Joel Berger, Karl Williamson, Lukas Mai, Martin Hasch, Matthew Horsfall, Nicholas Clark, Ricardo Signes, Ruslan Zakirov, Sergey Alekseev, Steffen Müller, Sullivan Beck, Sven Strickroth, Sébastien Aperghis-Tramoni, Tony Cook, Yves Orton.

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.