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perl5176delta - what is new for perl v5.17.6


This document describes differences between the 5.17.5 release and the 5.17.6 release.

If you are upgrading from an earlier release such as 5.17.4, first read perl5175delta, which describes differences between 5.17.4 and 5.17.5.

Core Enhancements

Character name aliases may now include non-Latin1-range characters

It is possible to define your own names for characters for use in \N{...}, charnames::vianame(), etc. These names can now be comprised of characters from the whole Unicode range. This allows for names to be in your native language, and not just English. Certain restrictions apply to the characters that may be used (you can't define a name that has punctuation in it, for example). See "CUSTOM ALIASES" in charnames.

New hash function Murmurhash-32 (v3)

We have switched Perl's hash function to use Murmurhash-32, and added build support for several other hash functions. This new function is expected to perform equivalently to the old one for shorter strings and is faster, potentially twice as fast, for hashing longer strings.

Incompatible Changes

An unknown character name in \N{...} is now a syntax error

Previously, it warned, and the Unicode REPLACEMENT CHARACTER was substituted. Unicode now recommends that this situation be a syntax error. Also, the previous behavior led to some confusing warnings and behaviors, and since the REPLACEMENT CHARACTER has no use other than as a stand-in for some unknown character, any code that has this problem is buggy.

Formerly deprecated characters in \N{} character name aliases are now errors.

Since v5.12.0, it has been deprecated to use certain characters in user-defined \N{...} character names. These now cause a syntax error. For example, it is now an error to begin a name with a digit, such as in

 my $undraftable = "\N{4F}";    # Syntax error!

or to have commas anywhere in the name. See "CUSTOM ALIASES" in charnames

Per process hash randomization

The seed used by Perl's hash function is now random. This means that the order which keys/values will be returned from functions like keys(), values(), and each() will differ from run to run.

This change was introduced to make Perl's hashes more robust to algorithmic complexity attacks, and also because we discovered that it exposes hash ordering dependency bugs and makes them easier to track down.

Toolchain maintainers might want to invest in additional infrastructure to test for things like this. Running tests several times in a row and then comparing results will make it easier to spot hash order dependencies in code. Authors are strongly encouraged not to expose the key order of Perl's hashes to insecure audiences.

PERL_HASH_SEED enviornment variable now takes a hex value

PERL_HASH_SEED no longer accepts an integer as a parameter, instead the value is expected to be a binary string encoded in hex. This is to make the infrastructure support hash seeds of arbitrary lengths which might exceed that of an integer. (SipHash uses a 16 byte seed).

Hash::Util::hash_seed() now returns a string

Hash::Util::hash_seed() now returns a string instead of an integer. This is to make the infrastructure support hash seeds of arbitrary lengths which might exceed that of an integer. (SipHash uses a 16 byte seed).

Output of PERL_HASH_SEED_DEBUG has been changed

The environment variable PERL_HASH_SEED_DEBUG now shows both the hash function perl was built with AND the seed, in hex in use for that process. Code parsing this output, should it exist, must change to accomodate the new format. Example of the new format:

    $ PERL_HASH_SEED_DEBUG=1 ./perl -e1

Performance Enhancements

  • Lists of lexical variable declarations (my($x, $y)) are now optimised down to a single op, and are hence faster than before.

  • A new C preprocessor define NO_TAINT_SUPPORT was added that, if set, disables Perl's taint support altogether. Using the -T or -t command line flags will cause a fatal error. Beware that both core tests as well as many a CPAN distribution's tests will fail with this change. On the upside, it provides a small performance benefit due to reduced branching.

    Do not enable this unless you know exactly what you are getting yourself into.

Modules and Pragmata

Updated Modules and Pragmata

  • Carp has been upgraded from version 1.27 to 1.28.

    Carp is no longer confused when caller returns undef for a package that has been deleted.

  • CPAN has been upgraded from version 1.98 to 1.99_51.

  • DynaLoader has been upgraded from version 1.16 to 1.17.

  • Env has been upgraded from version 1.03 to 1.04.

    Its SPLICE implementation no longer misbehaves in list context.

  • Module::CoreList has been upgraded from version 2.77 to 2.78.

  • Tie::Hash::NamedCapture has been upgraded from version 0.08 to 0.09.

Changes to Existing Documentation


  • *foo{NAME} and *foo{PACKAGE}, which have existed since perl 5.005, are now documented.

Platform Support

Discontinued Platforms


Support code relating to EPOC has been removed. EPOC was a family of operating systems developed by Psion for mobile devices. It was the predecessor of Symbian. The port was last updated in April 2002.

Platform-Specific Notes


Where possible, the case of filenames and command-line arguments is now preserved by enabling the CRTL features DECC$EFS_CASE_PRESERVE and DECC$ARGV_PARSE_STYLE at start-up time. The latter only takes effect when extended parse is enabled in the process from which Perl is run.


Building on WinCE is now possible once again, although more work is required to fully restore a clean build.

Internal Changes

  • The private Perl_croak_no_modify has had its context parameter removed. It is now has a void prototype. Users of the public API croak_no_modify remain unaffected.

  • Copy-on-write (shared hash key) scalars are no longer marked read-only. SvREADONLY returns false on such an SV, but SvIsCOW still returns true.

  • A new op type, OP_PADRANGE has been introduced. The perl peephole optimiser will, where possible, substitute a single padrange op for a pushmark followed by one or more pad ops, and possibly also skipping list and nextstate ops. In addition, the op can carry out the tasks associated with the RHS of a my(...) = @_ assignment, so those ops may be optimised away too.

Selected Bug Fixes

  • Uninitialized warnings mentioning hash elements would only mention the element name if it was not in the first bucket of the hash, due to an off-by-one error.

  • A regular expression optimizer bug could cause multiline "^" to behave incorrectly in the presence of line breaks, such that "/\n\n" =~ m#\A(?:^/$)#im would not match [perl #115242].

  • Failed fork in list context no longer currupts the stack. @a = (1, 2, fork, 3) used to gobble up the 2 and assign (1, undef, 3) if the fork call failed.

  • Numerous memory leaks have been fixed, mostly involving tied variables that die, regular expression character classes and code blocks, and syntax errors.

  • Assigning a regular expression (${qr//}) to a variable that happens to hold a floating point number no longer causes assertion failures on debugging builds.

  • Assigning a regular expression to a scalar containing a number no longer causes subsequent nummification to produce random numbers.

  • Assigning a regular expression to a magic variable no longer wipes away the magic. This is a regression from 5.10.

  • Assigning a regular expression to a blessed scalar no longer results in crashes. This is also a regression from 5.10.

  • Regular expression can now be assigned to tied hash and array elements with flattening into strings.

  • Nummifying a regular expression no longer results in an uninitialized warning.

  • Negative array indices no longer cause EXISTS methods of tied variables to be ignored. This is a regression from 5.12.

  • Negative array indices no longer result in crashes on arrays tied to non-objects.

  • $x = "(?{})"; /a++(?{})+$x/x no longer erroneous produces an error (just a warning, as expected). This was broken in 5.17.1.

  • $byte_overload .= $utf8 no longer results in doubly-encoded UTF8 if the left-hand scalar happened to have produced a UTF8 string the last time overloading was invoked.

  • goto &sub now uses the current value of @_, instead of using the array the subroutine was originally called with. This means local @_ = (...); goto &sub now works [perl #43077].

  • If a debugger is invoked recursively, it no longer stomps on its own lexical variables. Formerly under recursion all calls would share the same set of lexical variables [perl #115742].

  • *_{ARRAY} returned from a subroutine no longer spontaneously becomes empty.


Perl 5.17.6 represents approximately 5 weeks of development since Perl 5.17.5 and contains approximately 79,000 lines of changes across 460 files from 30 authors.

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 5.17.6:

Alexandr Ciornii, Brian Fraser, Chris 'BinGOs' Williams, Craig A. Berry, Dagfinn Ilmari Mannsåker, Daniel Dragan, David Golden, David Mitchell, Dominic Hargreaves, Eric Brine, Father Chrysostomos, Florian Ragwitz, Hugo van der Sanden, James E Keenan, Jerry D. Hedden, Jesse Luehrs, Karl Williamson, Lukas Mai, Nicholas Clark, Paul Johnson, Reini Urban, Ricardo Signes, Ruslan Zakirov, Shlomi Fish, Steffen Müller, Steve Hay, Tom Wyant, Tony Cook, Vadim Konovalov, 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 articles recently posted to the comp.lang.perl.misc newsgroup and the perl bug database at http://rt.perl.org/perlbug/ . There may also be information at http://www.perl.org/ , 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 perlbug@perl.org 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 perl5-security-report@perl.org. 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.