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perldelta - what is new for perl v5.23.1


This document describes differences between the 5.23.0 release and the 5.23.1 release.

If you are upgrading from an earlier release such as 5.22.0, first read perl5230delta, which describes differences between 5.22.0 and 5.23.0.

Core Enhancements ^

Integer shift (<< and >>) now more explicitly defined

Negative shifts are reverse shifts: left shift becomes right shift, and right shift becomes left shift.

Shifting by the number of bits in a native integer (or more) is zero, except when the "overshift" is right shifting a negative value under use integer, in which case the result is -1 (arithmetic shift).

Until now negative shifting and overshifting have been undefined because they have relied on whatever the C implementation happens to do. For example, for the overshift a common C behavior is "modulo shift":

  1 >> 64 == 1 >> (64 % 64) == 1 >> 0 == 1  # Common C behavior.

  # And the same for <<, while Perl now produces 0 for both.

Now these behaviors are well-defined under Perl, regardless of what the underlying C implementation does. Note, however, that you cannot escape the native integer width, you need to know how far left you can go. You can use for example:

  use Config;
  my $wordbits = $Config{uvsize} * 8;  # Or $Config{uvsize} << 3.

If you need a more bits on the left shift, you can use for example the bigint pragma, or the Bit::Vector module from CPAN.

Postfix dereferencing is no longer experimental

Using the postderef and postderef_qq features no longer emits a warning. Existing code that disables the experimental::postderef warning category that they previously used will continue to work. The postderef feature has no effect; all Perl code can use postfix dereferencing, regardless of what feature declarations are in scope. The 5.24 feature bundle now includes the postderef_qq feature.

printf and sprintf now allow reordered precision arguments

That is, sprintf '|%.*2$|', 2, 3 now returns |002|. This extends the existing reordering mechanism (which allows reordering for arguments that are used as format fields, widths, and vector separators).

Incompatible Changes ^

ASCII characters in variable names must now be all visible

It was legal until now on ASCII platforms for variable names to contain non-graphical ASCII control characters (ordinals 0 through 31, and 127, which are the C0 controls and DELETE). This usage has been deprecated since v5.20, and as of now causes a syntax error. The variables these names referred to are special, reserved by Perl for whatever use it may choose, now, or in the future. Each such variable has an alternative way of spelling it. Instead of the single non-graphic control character, a two character sequence beginning with a caret is used, like $^] and ${^GLOBAL_PHASE}. Details are at perlvar. It remains legal, though unwise and deprecated (raising a deprecation warning), to use certain non-graphic non-ASCII characters in variables names when not under use utf8. No code should do this, as all such variables are reserved by Perl, and Perl doesn't currently define any of them (but could at any time, without notice).

The autoderef feature has been removed

The experimental autoderef feature (which allowed calling push, pop, shift, unshift, splice, keys, values, and each on a scalar argument) has been deemed unsuccessful. It has now been removed; trying to use the feature (or to disable the experimental::autoderef warning it previously triggered) now yields an exception.

Modules and Pragmata ^

Updated Modules and Pragmata

Diagnostics ^

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

Platform Support ^

Platform-Specific Notes

  • The minimum supported version of VMS is now v7.3-2, released in 2003. As a side effect of this change, VAX is no longer supported as the terminal release of OpenVMS VAX was v7.3 in 2001.

Internal Changes ^

Selected Bug Fixes ^

Acknowledgements ^

Perl 5.23.1 represents approximately 4 weeks of development since Perl 5.23.0 and contains approximately 8,400 lines of changes across 320 files from 22 authors.

Excluding auto-generated files, documentation and release tools, there were approximately 5,000 lines of changes to 140 .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 5.23.1:

Aaron Crane, Aristotle Pagaltzis, Chas. Owens, Chris 'BinGOs' Williams, Craig A. Berry, Daniel Dragan, David Mitchell, Father Chrysostomos, Herbert Breunung, H.Merijn Brand, James E Keenan, Jarkko Hietaniemi, Karen Etheridge, Karl Williamson, Leon Timmermans, Matthew Horsfall, Max Maischein, Rafael Garcia-Suarez, Ricardo Signes, Stanislaw Pusep, Steve Hay, Tony Cook.

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.

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