You are viewing the version of this documentation from Perl 5.37.9. This is a development version of Perl.



perldelta - what is new for perl v5.37.9


This document describes differences between the 5.37.8 release and the 5.37.9 release.

If you are upgrading from an earlier release such as 5.37.7, first read perl5378delta, which describes differences between 5.37.7 and 5.37.8.

Core Enhancements

New class Feature

A new experimental syntax is now available for defining object classes, where per-instance data is stored in "field" variables that behave like lexicals.

use feature 'class';

class Point
    field $x;
    field $y;

    method zero { $x = $y = 0; }

This is described in more detail in perlclass. Notes on the internals of its implementation and other related details can be found in perlclassguts.

This remains a new and experimental feature, and is very much still under development. It will be the subject of much further addition, refinement and alteration in future releases. As it is experimental, it yields warnings in the experimental::class category. These can be silenced by a no warnings statement.

use feature 'class';
no warnings 'experimental::class';

REG_INF has been raised from 65,536 to 2,147,483,647

Many regex quantifiers used to be limited to U16_MAX in the past, but are now limited to I32_MAX, thus it is now possible to write /(?:word){1000000}/ for example. Note that doing so may cause the regex engine to run longer and use more memory.

New API functions optimize_optree and finalize_optree

There are two new API functions for operating on optree fragments, ensuring you can invoke the required parts of the optree-generation process that might otherwise not get invoked (e.g. when creating a custom LOGOP). To get access to these functions, you first need to set a #define to opt-in to using these functions.


These functions are closely tied to the internals of how the interpreter works, and could be altered or removed at any time if other internal changes make that necessary.

Incompatible Changes

(**{ ... }) removed from the regex engine.

This feature was released as part of 5.37.8, after some use and discussion it was seen as more problematic than understood at first and has been removed in 5.37.9. It was only ever present in a single development release and has never been released as part of a production perl, thus no deprecation cycle has been performed.


Use of ' as a package name separator is deprecated

Using ' as package separator in a variable named in a double-quoted string has warned since 5.28. It is now deprecated in both string interpolation and non-interpolated contexts, and will be removed in Perl 5.40.

Performance Enhancements

Modules and Pragmata

Updated Modules and Pragmata


New Documentation


Describes the new class feature.


Describes the internals of the new class feature.

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

Additionally, the following selected changes have been made:




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

Configuration and Compilation


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

Internal Changes

Selected Bug Fixes


Perl 5.37.9 represents approximately 4 weeks of development since Perl 5.37.8 and contains approximately 24,000 lines of changes across 360 files from 32 authors.

Excluding auto-generated files, documentation and release tools, there were approximately 8,400 lines of changes to 270 .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 5.37.9:

Alexander Nikolov, Alex Davies, Andrew Fresh, Aristotle Pagaltzis, Bartosz Jarzyna, Branislav Zahradník, Chad Granum, Craig A. Berry, Dagfinn Ilmari Mannsåker, Dan Jacobson, Elvin Aslanov, Håkon Hægland, Hugo van der Sanden, James E Keenan, Joe McMahon, Jonathan Stowe, Karen Etheridge, Karl Williamson, Kurt Fitzner, Leon Timmermans, Max Maischein, Nicholas Clark, Nicolas R, Paul Evans, Paul Marquess, Renee Baecker, Richard Leach, Scott Baker, Todd Rinaldo, Tomasz Konojacki, 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.