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perldelta - what is new for perl v5.15.7


This document describes differences between the 5.15.6 release and the 5.15.7 release.

If you are upgrading from an earlier release such as 5.15.5, first read perl5156delta, which describes differences between 5.15.5 and 5.15.6.

Core Enhancements

use charnames no longer needed for \N{name}

The charnames module is now automatically loaded when needed as if the :full and :short options had been specified. See charnames.

Improved performance for Unicode properties in regular expressions

Matching a code point against a Unicode property is now done via a binary search instead of linear. This means for example that the worst case for a 1000 item property is 10 probes instead of 1000. This inefficiency has been compensated for in the past by permanently storing in a hash the results of a given probe plus the results for the adjacent 64 code points, under the theory that near-by code points are likely to be searched for. A separate hash was used for each mention of a Unicode property in each regular expression. Thus, qr/\p{foo}abc\p{foo}/ would generate two hashes. Any probes in one instance would be unknown to the other, and the hashes could expand separately to be quite large if the regular expression were used on many different widely-separated code points. This can lead to running out of memory in extreme cases. Now, however, there is just one hash shared by all instances of a given property. This means that if \p{foo} is matched against "A" in one regular expression in a thread, the result will be known immediately to all regular expressions, and the relentless march of using up memory is slowed considerably.


Deprecated Modules


Version::Requirements is now DEPRECATED, use CPAN::Meta::Requirements, which is a drop-in replacement. It will be deleted from perl.git blead in v5.17.0.

Performance Enhancements

  • Version declarations with the use keyword (e.g., use 5.012) are now faster, as they enable features without loading feature.pm.

  • local $_ is faster now, as it no longer iterates through magic that it is not going to copy anyway.

Modules and Pragmata

Updated Modules and Pragmata

  • B::Deparse has been upgraded from version 1.10 to version 1.11.

    It now deparses open('random string') correctly. It used to omit the quotation marks, which did not work if the string were not a valid identifier [perl #91416].

    A similar bug also affected hash and array elements such as 'random string'->[0], which would deparse as $random string[0]. This has been fixed.

    Those same syntaxes used to drop the package name from variables beginning with a punctuation mark, as in "foo::]"->{$key}. This, too, has been fixed.

    B::Deparse no longer hangs when deparsing a program with stash circularities, such as BEGIN { *Acme::Acme:: = *Acme:: } [perl #91384].

    /$s[1]/ used to be deparsed as $s[1] if @s were a lexical variable [perl #81424]. Similarly, /$#s/ would be deparsed as $#s for both lexical and package variables. These has been fixed.

    The /applaud regular expression flags are no longer omitted.

    Feature hints are now deparsed with use feature rather than %^H assignments.

    A regression in 1.10 that caused ambient_pragmas to disable strict mode in obscure cases has been fixed.

    Strict mode is now fully deparsed, including subs and vars [perl #24027].

    The global variables $(, $| and $) are now deparsed with braces (i.e., ${(}) in regular expressions [perl #86060].

    continue blocks after for loops are now deparsed correctly, as they were back in 0.67 (included with Perl 5.8.5) [perl #108224]

  • CGI has been upgraded from version 3.58 to version 3.59.

    We no longer read from STDIN when the Content-Length is not set, preventing requests with no Content-Length from freezing in some cases. This is consistent with the CGI RFC 3875, and is also consistent with CGI::Simple. However, the old behavior may have been expected by some command-line uses of CGI.pm.

  • CPAN::Meta has been upgraded from version 2.112621 to version 2.113640.

    Version::Requirements has now been merged as CPAN::Meta::Requirements.

  • CPANPLUS has been upgraded from version 0.9113 to version 0.9116.

  • Data::Dumper has been upgraded from version 2.135_01 to version 2.135_03.

    It can now dump vstrings [perl #101162].

    The nameless typeglob (*{""}) is now dumped properly.

  • diagnostics has been upgraded from version 1.26 to version 1.27.

    See the entry for splain in the "Utility Changes" section, for the changes. The diagnostics module and the splain utility are actually one and the same.

  • ExtUtils::Packlist has been upgraded from version 1.45 to version 1.46.

    It no longer produces "used once" warnings when the read and write methods are called while the main program is still compiling (e.g., from within a BEGIN block) [perl #107410] [rt.cpan.org #50315].

  • ExtUtils::ParseXS has been upgraded from version 3.08 to version 3.12.

  • Locale::Maketext has been upgraded from version 1.21 to version 1.22.

  • Module::CoreList has been upgraded from version 2.59 to version 2.60.

  • Module::Pluggable has been upgraded from version 3.9 to version 4.0.

  • overload has been upgraded from version 1.16 to version 1.17.

    overload::Overloaded no longer calls can on the class, but uses another means to determine whether the object has overloading. It was never correct for it to call can, as overloading does not respect AUTOLOAD. So classes that autoload methods and implement can no longer have to account for overloading [perl #40333].

    A warning is now produced for invalid arguments. See "New Diagnostics".

  • perlfaq has been upgraded from version 5.0150036 to version 5.0150038.

  • PerlIO::scalar has been upgraded from version 0.12 to version 0.13.

    (This is the module that implements open $fh, '>', \$scalar.)

    It no longer assumes during seek that $scalar is a string internally. If it didn't crash, it was close to doing so [perl #92706]. Also, the internal print routine no longer assumes that the position set by seek is valid, but extends the string to that position, filling the intervening bytes (between the old length and the seek position) with nulls [perl #78980].

    Printing to an in-memory handle now works if the $scalar holds a reference, stringifying the reference before modifying it. References used to be treated as empty strings.

    Printing to an in-memory handle no longer crashes if the $scalar happens to hold a number internally, but no string buffer.

    Printing to an in-memory handle no longer creates scalars that confuse the regular expression engine [perl #108398].

  • Pod::Html has been upgraded from version 1.12 to 1.13

  • POSIX has been upgraded from version 1.27 to version 1.29.

    sigsuspend and pause now run signals handle before returning, as the whole point of these two functions is to wait until a signal has arrived, and then return after it has been triggered. Delayed, or "safe", signals were preventing that from happening, possibly resulting in race conditions [perl #107216]. POSIX::sleep is now a direct call into the underlying OS sleep function, instead of being a Perl wrapper on CORE::sleep. POSIX::dup2 now returns the correct value on Win32 (i.e. the file descriptor). POSIX::SigSet sigsuspend and sigpending and POSIX::pause now dispatch safe signals immediately before returning to their caller.

  • Pod::Perldoc has been upgraded from version 3.15_01 to version 3.15_15.

  • Term::UI has been upgraded from version 0.26 to version 0.30.

  • Tie::File has been upgraded from version 0.96 to version 0.98.

  • Unicode::UCD has been upgraded from version 0.37 to version 0.38. This changes the output of prop_invmap() for the Name_Alias property to reflect the changes that are planned for Unicode 6.1, so that there won't be a format change when upgrading to 6.1. Briefly, a second component of each alias is added that gives the type of alias it is. Examples are at "prop_invmap()" in Unicode::UCD.

  • Version::Requirements has been upgraded from version 0.101020 to version 0.101021.

    Version::Requirements is now DEPRECATED, use CPAN::Meta::Requirements, which is a drop-in replacement.


Changes to Existing Documentation


  • dbmopen treats a 0 mode as a special case, that prevents a nonexistent file from being created. This has been the case since Perl 5.000, but was never documented anywhere. Now the perlfunc entry mentions it [perl #90064].

  • The entry for split has been rewritten. It is now far clearer than before.

perlop and perlsyn

  • Documentation of the smartmatch operator has been reworked and moved from perlsyn to perlop where it belongs.

  • Documentation of the ellipsis statement (...) has been reworked and moved from perlop to 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

  • Cannot set tied @DB::args

    This error occurs when caller tries to set @DB::args but finds it tied. Before this error was added, it used to crash instead.

  • Cannot tie unreifiable array

    This error is part of a safety check that the tie operator does before tying a special array like @_. You should never see this message.

New Warnings

  • defined(@array) is deprecated

    The long-deprecated defined(@array) now also warns for package variables. Previously it only issued a warning for lexical variables.

  • Useless use of \E

    \E does nothing unless preceded by \Q, \L or \U.

  • overload arg '%s' is invalid

    This warning, in the "overload" category, is produced when the overload pragma is given an argument it doesn't recognize, presumably a mistyped operator.


  • "sort is now a reserved word"

    This error used to occur when sort was called without arguments, followed by ; or ). (E.g., sort; would die, but {sort} was OK.) This error message was added in Perl 3 to catch code like close(sort) which would no longer work. More than two decades later, this message is no longer appropriate. Now sort without arguments is always allowed, and returns an empty list, as it did in those cases where it was already allowed [perl #90030].

Utility Changes


  • splain no longer emits backtraces with the first line number repeated. This:

        Uncaught exception from user code:
                Cannot fwiddle the fwuddle at -e line 1.
         at -e line 1
                main::baz() called at -e line 1
                main::bar() called at -e line 1
                main::foo() called at -e line 1

    has become this:

        Uncaught exception from user code:
                Cannot fwiddle the fwuddle at -e line 1.
                main::baz() called at -e line 1
                main::bar() called at -e line 1
                main::foo() called at -e line 1
  • Some error messages consist of multiple lines that are listed as separate entries in perldiag. splain has been taught to find the separate entries in these cases, instead of simply failing to find the message.

Configuration and Compilation

  • The Pod files for the perl FAQ, perlxs, perlxstut and perldoc are once again correctly installed in the same directory as the other core Pods.


  • t/porting/utils.t now tests that various utility scripts compile cleanly. During development, this avoids the embarrassment of inadvertently pushing a commit which breaks code which isn't otherwise tested by the regression test suite. For example, installperl and installman, needed by make install, are tested here.

Internal Changes

  • There are now feature bundle hints in PL_hints ($^H) that version declarations use, to avoid having to load feature.pm. One setting of the hint bits indicates a "custom" feature bundle, which means that the entries in %^H still apply. feature.pm uses that.

    The HINT_FEATURE_MASK macro is defined in perl.h along with other hints. Other macros for setting and testing features and bundles are in the new feature.h. FEATURE_IS_ENABLED (which has moved to feature.h) is no longer used throughout the codebase, but more specific macros, e.g., FEATURE_SAY_IS_ENABLED, that are defined in feature.h.

  • lib/feature.pm is now a generated file, created by the new regen/feature.pl script, which also generates feature.h.

  • Tied arrays are now always AvREAL. If @_ or DB::args is tied, it is reified first, to make sure this is always the case.

Selected Bug Fixes

  • "b . COND" in the debugger has been fixed

    Breaking on the current line with b . COND was broken by previous work and has now been fixed.

  • Tying %^H

    Tying %^H no longer causes perl to crash or ignore the contents of %^H when entering a compilation scope [perl #106282].

  • ~ on vstrings

    The bitwise complement operator (and possibly other operators, too) when passed a vstring would leave vstring magic attached to the return value, even though the string had changed. This meant that version->new(~v1.2.3) would create a version looking like "v1.2.3" even though the string passed to version->new was actually "\376\375\374". This also caused B::Deparse to deparse ~v1.2.3 incorrectly, without the ~ [perl #29070].

  • Vstrings blowing away magic

    Assigning a vstring to a magic (e.g., tied, $!) variable and then assigning something else used to blow away all the magic. This meant that tied variables would come undone, $! would stop getting updated on failed system calls, $| would stop setting autoflush, and other mischief would take place. This has been fixed.

  • newHVhv and tied hashes

    The newHVhv XS function now works on tied hashes, instead of crashing or returning an empty hash.

  • Hashes will null elements

    It is possible from XS code to create hashes with elements that have no values. Perl itself sometimes creates such hashes, but they are rarely visible to Perl code. The hash element and slice operators used to crash when handling these in lvalue context. These have been fixed. They now produce a "Modification of non-creatable hash value attempted" error message.

  • No warning for open(foo::bar)

    When one writes open foo || die, which used to work in Perl 4, a "Precedence problem" warning is produced. This warning used erroneously to apply to fully-qualified bareword handle names not followed by ||. This has been corrected.

  • select and package aliasing

    After package aliasing (*foo:: = *bar::), select with 0 or 1 argument would sometimes return a name that could not be used to refer to the filehandle, or sometimes it would return undef even when a filehandle was selected. Now it returns a typeglob reference in such cases.

  • PerlIO::get_layers and tied variables

    PerlIO::get_layers no longer ignores FETCH on tied variables as it used to most of the time [perl #97956].

  • PerlIO::get_layers and numbers

    PerlIO::get_layers no longer ignores some arguments that it thinks are numeric, while treating others as filehandle names. It is now consistent for flat scalars (i.e., not references).

  • Lvalue subs and strict mode

    Lvalue sub calls that are not determined to be such at compile time (&$name or &{"name"}) are no longer exempt from strict refs if they occur in the last statement of an lvalue subroutine [perl #102486].

  • Non-lvalue sub calls in potentially lvalue context

    Sub calls whose subs are not visible at compile time, if they occurred in the last statement of an lvalue subroutine, would reject non-lvalue subroutines and die with "Can't modify non-lvalue subroutine call" [perl #102486].

    Non-lvalue sub calls whose subs are visible at compile time exhibited the opposite bug. If the call occurred in the last statement of an lvalue subroutine, there would be no error when the lvalue sub was called in lvalue context. Perl would blindly assign to the temporary value returned by the non-lvalue subroutine.

  • AUTOLOADing lvalue subs

    AUTOLOAD routines used to take precedence over the actual sub being called (i.e., when autoloading wasn't needed), for sub calls in lvalue or potential lvalue context, if the subroutine was not visible at compile time.

  • caller and tied @DB::args

    caller sets @DB::args to the subroutine arguments when called from the DB package. It used to crash when doing so if @DB::args happened to be tied. Now it croaks instead.

  • Tying @_

    Under debugging builds, this code:

      sub TIEARRAY{bless[]}
      sub {
        tie @_, "";

    use to produce an "av_reify called on tied array" warning. It doesn't any more.

  • Unrecognised switches on #! line

    If a switch, such as -x, that cannot occur on the #! line is used there, perl dies with "Can't emulate...".

    It used to produce the same message for switches that perl did not recognise at all, whether on the command line or the #! line.

    Now it produces the "Unrecognized switch" error message [perl #104288].

  • system and SIGCHLD

    system now temporarily blocks the SIGCHLD signal handler, to prevent the signal handler from stealing the exit status [perl #105700].

  • Deleting methods via delete

    Deletion of methods via delete $Class::{method} syntax used to update method caches if called in void context, but not scalar or list context. Now it always updates those caches.

  • Hash element deletion and destructors

    When hash elements are deleted in void context, the internal hash entry is now freed before the value is freed, to prevent destructors called by that latter freeing from seeing the hash in an inconsistent state. It was possible to cause double-frees if the destructor freed the hash itself [perl #100340].

  • (s)printf's %n formatting code

    The %n formatting code, which causes the number of characters to be assigned to the next argument to printf or sprintf now actually assigns the number of characters, instead of the number of bytes.

    It also works now with special lvalue functions like substr and with nonexistent hash and array elements [perl #3471, #103492].

  • Typeglobs and threads

    Typeglobs returned from threads are no longer cloned if the parent thread already has a glob with the same name. This means that returned subroutines will now assign to the right package variables [perl #107366].

  • local $_

    In Perl 5.14, local $_ was changed to create a new variable not tied to anything, even if $_ was tied before that. But, due to an oversight, it would still call FETCH once on a tied $_ before replacing it with the new variable. This has been fixed [perl #105912].

  • Returning tied variables

    When returning a value from a non-lvalue subroutine, Perl copies the value. Sometimes it cheats for the sake of speed, and does not copy the value if it makes no observable difference. This optimisation was erroneously allowing the copy to be skipped on tied variables, causing a difference in behaviour depending on the tied variable's reference count. This has been fixed [perl #95548].

  • {@a = sort} no longer crashes

    This particular piece of code (sort with no arguments assigned to an array, inside a block with no ;) started crashing in an earlier 5.15.x release. It has been fixed.

  • utf8::decode and read-only scalars

    utf8::decode now refuses to modify read-only scalars [perl #91850].

  • dbmopen with undefined mode

    dbmopen now only warns once, rather than three times, if the mode argument is undef [perl #90064].

  • Freeing an aggregate during list assignment

    If list assignment to a hash or array triggered destructors that freed the hash or array itself, a crash would ensue. This is no longer the case [perl #107440].

  • Confused internal bookkeeping with @ISA arrays

    Creating a weak reference to an @ISA array or accessing the array index ($#ISA) could result in confused internal bookkeeping for elements subsequently added to the @ISA array. For instance, creating a weak reference to the element itself could push that weak reference on to @ISA; and elements added after use of $#ISA would be ignored by method lookup [perl #85670].

  • DELETE on scalar ties

    Tying an element of %ENV or %^H and then deleting that element would result in a call to the tie object's DELETE method, even though tying the element itself is supposed to be equivalent to tying a scalar (the element is, of course, a scalar) [perl #67490].

  • Freeing $_ inside grep or map

    Freeing $_ inside a grep or map block or a code block embedded in a regular expression used to result in double frees [perl #92254, #92256].

  • Warnings with +=

    The += operator does not usually warn when the left-hand side is undef, but it was doing so for tied variables. This has been fixed [perl #44895].

  • Tying and autovivification

    When Perl autovivifies an element of a tied array or hash (which entails calling STORE with a new reference), it now calls FETCH immediately after the STORE, instead of assuming that FETCH would have returned the same reference. This can make it easier to implement tied objects [perl #35865, #43011].

  • @& and $&

    Mentioning a variable named "&" other than $& (i.e., @& or %&) no longer stops $& from working. The same applies to variables named "'" and "`" [perl #24237].

  • Stacked filetests

    -T and -B now work when stacked up with other filetest operators [perl #77388].

  • Filetests and stat buffers

    Perl keeps several internal variables to keep track of the last stat buffer, from which file(handle) it originated, what type it was, and whether the last stat succeeded.

    There were various cases where these could get out of synch, resulting in inconsistent or erratic behaviour in edge cases (every mention of -T applies to -B as well):

    • -T HANDLE, even though it does a stat, was not resetting the last stat type, so an lstat _ following it would merrily return the wrong results. Also, it was not setting the success status.

    • Freeing the handle last used by stat or a filetest could result in -T _ using an unrelated handle.

    • stat with an IO reference (as returned by *STDIO{IO}, for instance) would not reset the stat type.

    • stat with an IO reference was not recording the filehandle for -T _ to use.

    • The presence of fatal warnings could cause the stat buffer not to be reset for a filetest operator on an unopened filehandle or -l on any handle.

    • Fatal warnings would stop -T from setting $!.

    • When the last stat was on an unreadable file, -T _ is supposed to return undef, leaving the last stat buffer unchanged. But it was setting the stat type, causing lstat _ to stop working.

    • -T FILENAME was not resetting the internal stat buffers for unreadable files.

    These have all been fixed.

  • defined *{"!"}

    An earlier 5.15.x release caused this construct to stop the %! hash from working. Likewise defined *{"+"} and defined *{"-"} caused %+ and %-, respectively, to stop working. This has been fixed.

  • -T _ with no preceding stat

    This used to produce a confusing "uninitialized" warning, even though there is no visible uninitialized value to speak of.

  • stat HANDLE and fstat failures

    If the operating system's fstat function failed, stat would warn about an unopened handle, even though that was not the case. This has been fixed.

  • lstat IOREF

    lstat is documented to fall back to stat (with a warning) when given a filehandle. When passed an IO reference, it was actually doing the equivalent of stat _ and ignoring the handle.

  • Crashes with warnings

    Two warning messages that mention variable names started crashing in 5.15.5, but have been fixed [perl #106726, #107656].

  • Bitwise assignment operators and copy-on-write

    In 5.14.0, the bitwise assignment operators |=, ^= and &= started leaving the left-hand side undefined if it happened to be a copy-on-write string. This has been fixed [perl #108480].

  • Three problematic Unicode characters now work better in regex pattern matching under /i

    In the past, three Unicode characters: LATIN SMALL LETTER SHARP S, GREEK SMALL LETTER IOTA WITH DIALYTIKA AND TONOS, and GREEK SMALL LETTER UPSILON WITH DIALYTIKA AND TONOS, along with the sequences that they fold to (including "ss" in the case of LATIN SMALL LETTER SHARP S), did not properly match under /i. 5.14.0 fixed some of these cases, but introduced others, including a panic when one of the characters or sequences was used in the (?(DEFINE) regular expression predicate. The known bugs that were introduced in 5.14 have now been fixed; as well as some other edge cases that have never worked until now. All these involve using the characters and sequences outside bracketed character classes under /i. This closes [perl #98546].

    There remain known problems when using certain characters with multi-character folds inside bracketed character classes, including such constructs as qr/[\N{LATIN SMALL LETTER SHARP}a-z]/i. These remaining bugs are addressed in [perl #89774].


Perl 5.15.7 represents approximately 4 weeks of development since Perl 5.15.6 and contains approximately 51,000 lines of changes across 480 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 5.15.7:

Alberto Simões, Bo Lindbergh, Brian Fraser, Chris 'BinGOs' Williams, Craig A. Berry, Dave Rolsky, David Mitchell, Eric Brine, Father Chrysostomos, Florian Ragwitz, H.Merijn Brand, Hojung Youn, James E Keenan, Joel Berger, Joshua ben Jore, Karl Williamson, Leon Timmermans, Matthew Horsfall, Michael Witten, Nicholas Clark, Reini Urban, Ricardo Signes, Shlomi Fish, Steffen Müller, Steffen Schwigon, Todd Rinaldo, Tom Christiansen, Tom Hukins, Tony Cook, Ævar Arnfjörð Bjarmason.

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.