perldelta - what is new for perl v5.37.6


This document describes differences between the 5.37.5 release and the 5.37.6 release.

If you are upgrading from an earlier release such as 5.37.4, first read perl5375delta, which describes differences between 5.37.4 and 5.37.5.

Incompatible Changes


Starting in this release, if the input string is undef, it remains undef. Previously it would be changed into a defined, zero-length string.

Modules and Pragmata

New Modules and Pragmata

  • Support for RFC-18, use feature "module_true"; has been added to the default feature bundle for 5.37 and later. It may also be used explicitly. When enabled inside of a module the module does not need to return true explicitly, and in fact the return will be forced to a simple true value regardless of what it originally was.

Updated Modules and Pragmata

  • Storable has been upgraded from version 3.27 to 3.28.

    The STORABLE_freeze callback now supports regular expression objects.

    The error message when Storable attempts to call STORABLE_freeze for an unsupported type such as GLOBs has been improved from "Unexpected object type (8) in store_hook()" to indicate the perl object type and the class.


Changes to Existing Documentation

We have attempted to update the documentation to reflect the changes listed in this document. If you find any we have missed, open an issue at


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

  • Various warnings that used to produce parenthesized hints underneath the main warning message and after its "location data" were chanaged to put the hint inline with the main message. For instance:

     Bareword found where operator expected at -e line 1, near "foo bar"
         (Do you need to predeclare foo?)

    will now look like this but as one line:

     Bareword found where operator expected (Do you need to predeclare
     foo?) at -e line 1, near "foo bar"

    as a result such warnings will no longer trigger $SIG{__WARN__} twice, and the hint will be visible when fatal warnings is in effect.

Utility Changes



Configuration and Compilation

  • I32/U32 formatting symbols added in 5.37.2, I32df, U32xf, etc. used to be defined in perl.h using preprocessor conditionals. They are now determined in Configure and defined in config.h.


  • Tests were added and changed to reflect the other additions and changes in this release.

Platform Support


stat() now works on AF_UNIX socket files. [github #20204]

readlink() now returns the PrintName from a symbolic link reparse point instead of the SubstituteName, which should make it better match the name the link was created with. [github #20271]

Internal Changes

  • A new magic type PERL_MAGIC_extvalue has been added. This is available for use like PERL_MAGIC_ext, but is a value magic: upon localization the new value will not be magical.

  • The SSNEW(), SSNEWt(), SSNEWa() and SSNEWat() APIs now return a SSize_t value. The SSPTR() and SSPTRt() macros now expect a SSize_t parameter, and enforce that on debugging builds. [github #20411]

  • Filenames in cops are now refcounted under threads. Under threads we were copying the filenames into each opcode. This is because in theory opcodes created in one thread can be destroyed in another. The change adds a new struct/type RCPV, which is a refcounted string using shared memory. This is implemented in such a way that code that previously used a char * can continue to do so, as the refcounting data is located a specific offset before the char * pointer itself.

    [commit 6760f691a95ab3a37fd59212795de2b1a7cf7888]

-=head1 Selected Bug Fixes

  • Fixes to memory handling for PL_splitstr:

    • If a thread was created the allocated string would be freed twice.

    • If two -F switches were supplied the memory allocated for the first switch wouldn't be freed.

Known Problems


Errata From Previous Releases



Perl 5.37.6 represents approximately 4 weeks of development since Perl 5.37.5 and contains approximately 16,000 lines of changes across 230 files from 26 authors.

Excluding auto-generated files, documentation and release tools, there were approximately 12,000 lines of changes to 130 .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 5.37.6:

Alexander Nikolov, Andreas König, Aristotle Pagaltzis, Bram, Branislav Zahradník, chromatic, David Mitchell, E. Choroba, Elvin Aslanov, Felipe Gasper, Graham Knop, James E Keenan, Joe McMahon, Johan Vromans, Karl Williamson, Leon Timmermans, Mattia Barbon, Max Maischein, Paul Evans, Ricardo Signes, Richard Leach, TAKAI Kousuke, Todd Rinaldo, Tom Stellard, 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 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.