perldelta - what is new for perl v5.28.0


This document describes differences between the 5.26.0 release and the 5.28.0 release.

If you are upgrading from an earlier release such as 5.24.0, first read perl5260delta, which describes differences between 5.24.0 and 5.26.0.

Core Enhancements

Unicode 10.0 is supported

A list of changes is at

delete on key/value hash slices

delete can now be used on key/value hash slices, returning the keys along with the deleted values. [perl #131328]

Experimentally, there are now alphabetic synonyms for some regular expression assertions

If you find it difficult to remember how to write certain of the pattern assertions, there are now alphabetic synonyms.

------                 ------------
(?=...)        (*pla:...) or (*positive_lookahead:...)
(?!...)        (*nla:...) or (*negative_lookahead:...)
(?<=...)       (*plb:...) or (*positive_lookbehind:...)
(?<!...)       (*nlb:...) or (*negative_lookbehind:...)
(?>...)        (*atomic:...)

These are considered experimental, so using any of these will raise (unless turned off) a warning in the experimental::alpha_assertions category.

Mixed Unicode scripts are now detectable

A mixture of scripts, such as Cyrillic and Latin, in a string is often the sign of a spoofing attack. A new regular expression construct now allows for easy detection of these. For example, you can say

qr/(*script_run: \d+ \b )/x

And the digits matched will all be from the same set of 10. You won't get a look-alike digit from a different script that has a different value than what it appears to be.


qr/(*sr: \b \w+ \b )/x

makes sure that all the characters come from the same script.

You can also combine script runs with (?>...) (or *atomic:...)).

Instead of writing:


you can now run:

# or

This is considered experimental, so using it will raise (unless turned off) a warning in the experimental::script_run category.

See "Script Runs" in perlre.

In-place editing with perl -i is now safer

Previously in-place editing (perl -i) would delete or rename the input file as soon as you started working on a new file.

Without backups this would result in loss of data if there was an error, such as a full disk, when writing to the output file.

This has changed so that the input file isn't replaced until the output file has been completely written and successfully closed.

This works by creating a work file in the same directory, which is renamed over the input file once the output file is complete.


[perl #127663]

Initialisation of aggregate state variables

A persistent lexical array or hash variable can now be initialized, by an expression such as state @a = qw(x y z). Initialization of a list of persistent lexical variables is still not possible.

Full-size inode numbers

On platforms where inode numbers are of a type larger than perl's native integer numerical types, stat will preserve the full content of large inode numbers by returning them in the form of strings of decimal digits. Exact comparison of inode numbers can thus be achieved by comparing with eq rather than ==. Comparison with ==, and other numerical operations (which are usually meaningless on inode numbers), work as well as they did before, which is to say they fall back to floating point, and ultimately operate on a fairly useless rounded inode number if the real inode number is too big for the floating point format.

The sprintf %j format size modifier is now available with pre-C99 compilers

The actual size used depends on the platform, so remains unportable.

Close-on-exec flag set atomically

When opening a file descriptor, perl now generally opens it with its close-on-exec flag already set, on platforms that support doing so. This improves thread safety, because it means that an exec initiated by one thread can no longer cause a file descriptor in the process of being opened by another thread to be accidentally passed to the executed program.

Additionally, perl now sets the close-on-exec flag more reliably, whether it does so atomically or not. Most file descriptors were getting the flag set, but some were being missed.

String- and number-specific bitwise ops are no longer experimental

The new string-specific (&. |. ^. ~.) and number-specific (& | ^ ~) bitwise operators introduced in Perl 5.22 that are available within the scope of use feature 'bitwise' are no longer experimental. Because the number-specific ops are spelled the same way as the existing operators that choose their behaviour based on their operands, these operators must still be enabled via the "bitwise" feature, in either of these two ways:

use feature "bitwise";

use v5.28; # "bitwise" now included

They are also now enabled by the -E command-line switch.

The "bitwise" feature no longer emits a warning. Existing code that disables the "experimental::bitwise" warning category that the feature previously used will continue to work.

One caveat that module authors ought to be aware of is that the numeric operators now pass a fifth TRUE argument to overload methods. Any methods that check the number of operands may croak if they do not expect so many. XS authors in particular should be aware that this:

SV *
bitop_handler (lobj, robj, swap)

may need to be changed to this:

SV *
bitop_handler (lobj, robj, swap, ...)

Locales are now thread-safe on systems that support them

These systems include Windows starting with Visual Studio 2005, and in POSIX 2008 systems.

The implication is that you are now free to use locales and change them in a threaded environment. Your changes affect only your thread. See "Multi-threaded operation" in perllocale

New read-only predefined variable ${^SAFE_LOCALES}

This variable is 1 if the Perl interpreter is operating in an environment where it is safe to use and change locales (see perllocale.) This variable is true when the perl is unthreaded, or compiled in a platform that supports thread-safe locale operation (see previous item).


[CVE-2017-12837] Heap buffer overflow in regular expression compiler

Compiling certain regular expression patterns with the case-insensitive modifier could cause a heap buffer overflow and crash perl. This has now been fixed. [perl #131582]

[CVE-2017-12883] Buffer over-read in regular expression parser

For certain types of syntax error in a regular expression pattern, the error message could either contain the contents of a random, possibly large, chunk of memory, or could crash perl. This has now been fixed. [perl #131598]

[CVE-2017-12814] $ENV{$key} stack buffer overflow on Windows

A possible stack buffer overflow in the %ENV code on Windows has been fixed by removing the buffer completely since it was superfluous anyway. [perl #131665]

Default Hash Function Change

Perl 5.28.0 retires various older hash functions which are not viewed as sufficiently secure for use in Perl. We now support four general purpose hash functions, Siphash (2-4 and 1-3 variants), and Zaphod32, and StadtX hash. In addition we support SBOX32 (a form of tabular hashing) for hashing short strings, in conjunction with any of the other hash functions provided.

By default Perl is configured to support SBOX hashing of strings up to 24 characters, in conjunction with StadtX hashing on 64 bit builds, and Zaphod32 hashing for 32 bit builds.

You may control these settings with the following options to Configure:


To disable SBOX hashing you can use


And to set the maximum length to use SBOX32 hashing on with:


The maximum length allowed is 256. There probably isn't much point in setting it higher than the default.

Incompatible Changes

Subroutine attribute and signature order

The experimental subroutine signatures feature has been changed so that subroutine attributes must now come before the signature rather than after. This is because attributes like :lvalue can affect the compilation of code within the signature, for example:

sub f :lvalue ($a = do { $x = "abc"; return substr($x,0,1)}) { ...}

Note that this the second time they have been flipped:

sub f :lvalue ($a, $b) { ... }; # 5.20; 5.28 onwards
sub f ($a, $b) :lvalue { ... }; # 5.22 - 5.26

Comma-less variable lists in formats are no longer allowed

Omitting the commas between variables passed to formats is no longer allowed. This has been deprecated since Perl 5.000.

The :locked and :unique attributes have been removed

These have been no-ops and deprecated since Perl 5.12 and 5.10, respectively.

\N{} with nothing between the braces is now illegal

This has been deprecated since Perl 5.24.

Opening the same symbol as both a file and directory handle is no longer allowed

Using open() and opendir() to associate both a filehandle and a dirhandle to the same symbol (glob or scalar) has been deprecated since Perl 5.10.

Use of bare << to mean <<"" is no longer allowed

Use of a bare terminator has been deprecated since Perl 5.000.

Setting $/ to a reference to a non-positive integer no longer allowed

This used to work like setting it to undef, but has been deprecated since Perl 5.20.

Unicode code points with values exceeding IV_MAX are now fatal

This was deprecated since Perl 5.24.

The B::OP::terse method has been removed

Use B::Concise::b_terse instead.

Use of inherited AUTOLOAD for non-methods is no longer allowed

This was deprecated in Perl 5.004.

Use of strings with code points over 0xFF is not allowed for bitwise string operators

Code points over 0xFF do not make sense for bitwise operators and such an operation will now croak, except for a few remaining cases. See perldeprecation.

This was deprecated in Perl 5.24.

Setting ${^ENCODING} to a defined value is now illegal

This has been deprecated since Perl 5.22 and a no-op since Perl 5.26.

Backslash no longer escapes colon in PATH for the -S switch

Previously the -S switch incorrectly treated backslash ("\") as an escape for colon when traversing the PATH environment variable. [perl #129183]

the -DH (DEBUG_H) misfeature has been removed

On a perl built with debugging support, the H flag to the -D debugging option has been removed. This was supposed to dump hash values, but has been broken for many years.

Yada-yada is now strictly a statement

By the time of its initial stable release in Perl 5.12, the ... (yada-yada) operator was explicitly intended to serve as a statement, not an expression. However, the original implementation was confused on this point, leading to inconsistent parsing. The operator was accidentally accepted in a few situations where it did not serve as a complete statement, such as

... . "foo";
... if $a < $b;

The parsing has now been made consistent, permitting yada-yada only as a statement. Affected code can use do{...} to put a yada-yada into an arbitrary expression context.

Sort algorithm can no longer be specified

Since Perl 5.8, the sort pragma has had subpragmata _mergesort, _quicksort, and _qsort that can be used to specify which algorithm perl should use to implement the sort builtin. This was always considered a dubious feature that might not last, hence the underscore spellings, and they were documented as not being portable beyond Perl 5.8. These subpragmata have now been deleted, and any attempt to use them is an error. The sort pragma otherwise remains, and the algorithm-neutral stable subpragma can be used to control sorting behaviour. [perl #119635]

Over-radix digits in floating point literals

Octal and binary floating point literals used to permit any hexadecimal digit to appear after the radix point. The digits are now restricted to those appropriate for the radix, as digits before the radix point always were.

Return type of unpackstring()

The return types of the C API functions unpackstring() and unpack_str() have changed from I32 to SSize_t, in order to accommodate datasets of more than two billion items.


Use of vec on strings with code points above 0xFF is deprecated

Such strings are represented internally in UTF-8, and vec is a bit-oriented operation that will likely give unexpected results on those strings.

Some uses of unescaped "{" in regexes are no longer fatal

Perl 5.26.0 fatalized some uses of an unescaped left brace, but an exception was made at the last minute, specifically crafted to be a minimal change to allow GNU Autoconf to work. That tool is heavily depended upon, and continues to use the deprecated usage. Its use of an unescaped left brace is one where we have no intention of repurposing "{" to be something other than itself.

That exception is now generalized to include various other such cases where the "{" will not be repurposed.

Note that these uses continue to raise a deprecation message.

Use of unescaped "{" immediately after a "(" in regular expression patterns is deprecated

Using unescaped left braces is officially deprecated everywhere, but it is not enforced in contexts where their use does not interfere with expected extensions to the language. A deprecation is added in this release when the brace appears immediately after an opening parenthesis. Before this, even if the brace was part of a legal quantifier, it was not interpreted as such, but as the literal characters, unlike other quantifiers that follow a "(" which are considered errors. Now, their use will raise a deprecation message, unless turned off.

Assignment to $[ will be fatal in Perl 5.30

Assigning a non-zero value to $[ has been deprecated since Perl 5.12, but was never given a deadline for removal. This has now been scheduled for Perl 5.30.

hostname() won't accept arguments in Perl 5.32

Passing arguments to Sys::Hostname::hostname() was already deprecated, but didn't have a removal date. This has now been scheduled for Perl 5.32. [perl #124349]

Module removals

The following modules will be removed from the core distribution in a future release, and will at that time need to be installed from CPAN. Distributions on CPAN which require these modules will need to list them as prerequisites.

The core versions of these modules will now issue "deprecated"-category warnings to alert you to this fact. To silence these deprecation warnings, install the modules in question from CPAN.

Note that these are (with rare exceptions) fine modules that you are encouraged to continue to use. Their disinclusion from core primarily hinges on their necessity to bootstrapping a fully functional, CPAN-capable Perl installation, not usually on concerns over their design.

Locale::Codes and its associated Country, Currency and Language modules

Performance Enhancements

Modules and Pragmata

Key highlights in this release across several modules:

Removal of use vars

The usage of use vars has been discouraged since the introduction of our in Perl 5.6.0. Where possible the usage of this pragma has now been removed from the Perl source code.

This had a slight effect (for the better) on the output of WARNING_BITS in B::Deparse.

Use of DynaLoader changed to XSLoader in many modules

XSLoader is more modern, and most modules already require perl 5.6 or greater, so no functionality is lost by switching. In some cases, we have also made changes to the local implementation that may not be reflected in the version on CPAN due to a desire to maintain more backwards compatibility.

Updated Modules and Pragmata

Removed Modules and Pragmata


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, send email to

Additionally, the following selected changes have been made:



















perlfunc, perlop, perlsyn


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



For the past few years we have released perl using three different archive formats: bzip (.bz2), LZMA2 (.xz) and gzip (.gz). Since xz compresses better and decompresses faster, and gzip is more compatible and uses less memory, we have dropped the .bz2 archive format with this release. (If this poses a problem, do let us know; see "Reporting Bugs", below.)

Platform Support

Discontinued Platforms

PowerUX / Power MAX OS

Compiler hints and other support for these apparently long-defunct platforms has been removed.

Platform-Specific Notes


Compilation on CentOS 5 is now fixed.


A build with the quadmath library can now be done on Cygwin.


Perl now correctly uses reentrant functions, like asctime_r, on versions of Darwin that have support for them.


FreeBSD's /usr/share/mk/ specifies -O2 for architectures other than ARM and MIPS. By default, perl is now compiled with the same optimization levels.


Several fix-ups for, marking function VMS has (or doesn't have).

CRTL features can now be set by embedders before invoking Perl by using the decc$feature_set and decc$feature_set_value functions. Previously any attempt to set features after image initialization were ignored.

  • Support for compiling perl on Windows using Microsoft Visual Studio 2017 (containing Visual C++ 14.1) has been added.

  • Visual C++ compiler version detection has been improved to work on non-English language systems.

  • We now set $Config{libpth} correctly for 64-bit builds using Visual C++ versions earlier than 14.1.

Internal Changes

Selected Bug Fixes


Perl 5.28.0 represents approximately 13 months of development since Perl 5.26.0 and contains approximately 730,000 lines of changes across 2,200 files from 77 authors.

Excluding auto-generated files, documentation and release tools, there were approximately 580,000 lines of changes to 1,300 .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 5.28.0:

Aaron Crane, Abigail, Ævar Arnfjörð Bjarmason, Alberto Simões, Alexandr Savca, Andrew Fresh, Andy Dougherty, Andy Lester, Aristotle Pagaltzis, Ask Bjørn Hansen, Chris 'BinGOs' Williams, Craig A. Berry, Dagfinn Ilmari Mannsåker, Dan Collins, Daniel Dragan, David Cantrell, David Mitchell, Dmitry Ulanov, Dominic Hargreaves, E. Choroba, Eric Herman, Eugen Konkov, Father Chrysostomos, Gene Sullivan, George Hartzell, Graham Knop, Harald Jörg, H.Merijn Brand, Hugo van der Sanden, Jacques Germishuys, James E Keenan, Jarkko Hietaniemi, Jerry D. Hedden, J. Nick Koston, John Lightsey, John Peacock, John P. Linderman, John SJ Anderson, Karen Etheridge, Karl Williamson, Ken Brown, Ken Cotterill, Leon Timmermans, Lukas Mai, Marco Fontani, Marc-Philip Werner, Matthew Horsfall, Neil Bowers, Nicholas Clark, Nicolas R., Niko Tyni, Pali, Paul Marquess, Peter John Acklam, Reini Urban, Renee Baecker, Ricardo Signes, Robin Barker, Sawyer X, Scott Lanning, Sergey Aleynikov, Shirakata Kentaro, Shoichi Kaji, Slaven Rezic, Smylers, Steffen Müller, Steve Hay, Sullivan Beck, Thomas Sibley, Todd Rinaldo, Tomasz Konojacki, Tom Hukins, Tom Wyant, Tony Cook, Vitali Peil, Yves Orton, Zefram.

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 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 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.