You are viewing the version of this documentation from Perl 5.37.5. This is a development version of Perl.



perldelta - what is new for perl v5.37.5


This document describes differences between the 5.37.4 release and the 5.37.5 release.

If you are upgrading from an earlier release such as 5.37.3, first read perl5374delta, which describes differences between 5.37.3 and 5.37.4.


Perl 5.28 introduced "thread-safe" locales on systems that supported them, namely modern Windows, and systems supporting POSIX 2008 locale operations. These systems accomplish this by having per-thread locales, while continuing to support the older global locale operations for code that doesn't take the steps necessary to use the newer per-thread ones.

It turns out that some POSIX 2008 platforms have or have had buggy implementations, which forced perl to not use them. The ${^SAFE_LOCALES} scalar variable contains 0 or 1 to indicate whether or not the current platform is considered by perl to have a working thread-safe implementation. Some implementations have been fixed already, but FreeBSD and Cygwin have been newly discovered to be sufficiently buggy that the thread-safe operations are no longer used by perl, starting in this release. Hence, ${^SAFE_LOCALES} is now 0 for them. Older versions of perl can be configured to avoid these buggy implementations by adding the Configure option -DNO_POSIX_2008_LOCALE.

And 5.37.5 fixes a bug in all previous perls that led to locales not being fully thread-safe. The first thread that finishes caused the main thread (named thread0) to revert to the global locale in effect at startup, discarding whatever the thread's locale had been previously set to. If any other thread had switched to the global locale by calling switch_to_global_locale() in XS code, those threads would all share the global locale, and thread0 would not be thread-safe.

Core Enhancements

Unicode 15.0 is supported

See for details.

Performance Enhancements

Modules and Pragmata

Updated Modules and Pragmata


New Documentation

"Writing safer macros" in perlhacktips

"Choosing good symbol names" in perlhacktips

Configuration and Compilation



Perl 5.37.5 represents approximately 4 weeks of development since Perl 5.37.4 and contains approximately 40,000 lines of changes across 220 files from 24 authors.

Excluding auto-generated files, documentation and release tools, there were approximately 31,000 lines of changes to 97 .pm, .t, .c and .h files.

Perl continues to flourish into its fourth decade thanks to a vibrant community of users and developers. The following people are known to have contributed the improvements that became Perl 5.37.5:

Alex, Bram, Craig A. Berry, Dan Book, Elvin Aslanov, Felipe Gasper, Graham Knop, James E Keenan, Karen Etheridge, Karl Williamson, Kenichi Ishigaki, Leon Timmermans, Nathan Mills, Nicolas R, Paul Evans, Peter John Acklam, Philippe Bruhat (BooK), Ricardo Signes, Richard Leach, TAKAI Kousuke, Todd Rinaldo, Tony Cook, Unicode Consortium, 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 perl bug database at There may also be information at, the Perl Home Page.

If you believe you have an unreported bug, please open an issue at Be sure to trim your bug down to a tiny but sufficient test case.

If the bug you are reporting has security implications which make it inappropriate to send to a public issue tracker, then see "SECURITY VULNERABILITY CONTACT INFORMATION" in perlsec for details of how to report the issue.

Give Thanks

If you wish to thank the Perl 5 Porters for the work we had done in Perl 5, you can do so by running the perlthanks program:


This will send an email to the Perl 5 Porters list with your show of thanks.


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.