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perl5211delta - what is new for perl v5.21.1


This document describes differences between the 5.21.0 release and the 5.21.1 release.

If you are upgrading from an earlier release such as 5.20.0, first read perl5210delta, which describes differences between 5.20.0 and 5.21.0.


This release removes a number of previously deprecated constructs, many that have been around for a long time. Please see "Incompatible Changes" for more information.

Core Enhancements

Unicode 7.0 is now supported

For details on what is in this release, see

Experimental C Backtrace API

Starting from Perl 5.21.1, on some platforms Perl supports retrieving the C level backtrace (similar to what symbolic debuggers like gdb do).

The backtrace returns the stack trace of the C call frames, with the symbol names (function names), the object names (like "perl"), and if it can, also the source code locations (file:line).

The supported platforms are Linux and OS X (some *BSD might work at least partly, but they have not yet been tested).

The feature needs to be enabled with Configure -Dusecbacktrace.

Also included is a C API to retrieve backtraces.

See "C backtrace" in perlhacktips for more information.

qr/foo/x now ignores any Unicode pattern white space

The /x regular expression modifier allows the pattern to contain white space and comments, both of which are ignored, for improved readability. Until now, not all the white space characters that Unicode designates for this purpose were handled. The additional ones now recognized are U+0085 NEXT LINE, U+200E LEFT-TO-RIGHT MARK, U+200F RIGHT-TO-LEFT MARK, U+2028 LINE SEPARATOR, and U+2029 PARAGRAPH SEPARATOR.

use locale can restrict which locale categories are affected

It is now possible to pass a parameter to use locale to specify a subset of locale categories to be locale-aware, with the remaining ones unaffected. See "The "use locale" pragma" in perllocale for details.

Incompatible Changes

\N{} with a sequence of multiple spaces is now a fatal error.

This has been deprecated since v5.18.

In double-quotish \cX, X must now be a printable ASCII character

In prior releases, failure to do this raised a deprecation warning.

Splitting the tokens (? and (* in regular expressions is now a fatal compilation error.

These had been deprecated since v5.18.

5 additional characters are treated as white space under /x in regex patterns (unless escaped)

The use of these characters with /x outside bracketed character classes and when not preceded by a backslash has raised a deprecation warning since v5.18. Now they will be ignored. See "qr/foo/x" for the list of the five characters.

Comment lines within (?[ ]) now are ended only by a \n

(?[ ]) is an experimental feature, introduced in v5.18. It operates as if /x is always enabled. But there was a difference, comment lines (following a # character) were terminated by anything matching \R which includes all vertical whitespace, such as form feeds. For consistency, this is now changed to match what terminates comment lines outside (?[ ]), namely a \n (even if escaped), which is the same as what terminates a heredoc string and formats.

Omitting % and @ on hash and array names is no longer permitted

Really old Perl let you omit the @ on array names and the % on hash names in some spots. This has issued a deprecation warning since Perl 5.0, and is no longer permitted.

"$!" text is now in English outside "use locale" scope

Previously, the text, unlike almost everything else, always came out based on the current underlying locale of the program. (Also affected on some systems is "$^E".) For programs that are unprepared to handle locale, this can cause garbage text to be displayed. It's better to display text that is translatable via some tool than garbage text which is much harder to figure out.

"$!" text will be returned in UTF-8 when appropriate

The stringification of $! and $^E will have the UTF-8 flag set when the text is actually non-ASCII UTF-8. This will enable programs that are set up to be locale-aware to properly output messages in the user's native language. Code that needs to continue the 5.20 and earlier behavior can do the stringification within the scopes of both 'use bytes' and 'use locale ":messages". No other Perl operations will be affected by locale; only $! and $^E stringification. The 'bytes' pragma causes the UTF-8 flag to not be set, just as in previous Perl releases. This resolves [perl #112208].

Support for ?PATTERN? without explicit operator has been removed

Starting regular expressions matching only once directly with the question mark delimiter is now a syntax error, so that the question mark can be available for use in new operators. Write m?PATTERN? instead, explicitly using the m operator: the question mark delimiter still invokes match-once behaviour.

defined(@array) and defined(%hash) are now fatal errors

These have been deprecated since v5.6.1 and have raised deprecation warnings since v5.16.

Using a hash or an array as a reference are now fatal errors.

For example, %foo->{"bar"} now causes a fatal compilation error. These have been deprecated since before v5.8, and have raised deprecation warnings since then.


Using a NO-BREAK space in a character alias for \N{...} is now deprecated

This non-graphic character is essentially indistinguishable from a regular space, and so should not be allowed. See "CUSTOM ALIASES" in charnames.

A literal "{" should now be escaped in a pattern

If you want a literal left curly bracket (also called a left brace) in a regular expression pattern, you should now escape it by either preceding it with a backslash ("\{") or enclosing it within square brackets "[{]", or by using \Q; otherwise a deprecation warning will be raised. This was first announced as forthcoming in the v5.16 release; it will allow future extensions to the language to happen.

Performance Enhancements

Modules and Pragmata

Updated Modules and Pragmata


Changes to Existing Documentation







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 Errors

New Warnings

Changes to Existing Diagnostics

Utility Changes


Configuration and Compilation

Platform Support

Discontinued Platforms


NeXTSTEP was proprietary OS bundled with NeXT's workstations in the early to mid 90's; OPENSTEP was an API specification that provided a NeXTSTEP-like environment on a non-NeXTSTEP system. Both are now long dead, so support for building Perl on them has been removed.

Platform-Specific Notes


On OpenBSD, Perl will now default to using the system malloc due to the security features it provides. Perl's own malloc wrapper has been in use since v5.14 due to performance reasons, but the OpenBSD project believes the tradeoff is worth it and would prefer that users who need the speed specifically ask for it.

[perl #122000].

Internal Changes

Selected Bug Fixes


Perl 5.21.1 represents approximately 3 weeks of development since Perl 5.21.0 and contains approximately 240,000 lines of changes across 680 files from 37 authors.

Excluding auto-generated files, documentation and release tools, there were approximately 150,000 lines of changes to 420 .pm, .t, .c and .h files.

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 5.21.1:

Alex Solovey, Andrew Fresh, Andy Dougherty, Brian Fraser, Chris 'BinGOs' Williams, Craig A. Berry, Dagfinn Ilmari Mannsåker, Daniel Dragan, Darin McBride, David Mitchell, Doug Bell, H.Merijn Brand, James E Keenan, Jarkko Hietaniemi, kafka, Karen Etheridge, Karl Williamson, Leon Timmermans, Matthew Horsfall, Michael Bunk, Nicholas Clark, Niels Thykier, Norman Koch, Peter John Acklam, Pierre Bogossian, Reini Urban, Ricardo Signes, Rob Hoelz, Shlomi Fish, Smylers, Steffen Müller, Steve Hay, Sullivan Beck, Thomas Sibley, Todd Rinaldo, 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 . 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.