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perl56delta - what's new for perl v5.6.0


This document describes differences between the 5.005 release and the 5.6.0 release.

New tests

Compatibility tests for sub : attrs vs the older use attrs.
Tests for new environment scalar capability (e.g., use Env qw($BAR);).
Tests for new environment array capability (e.g., use Env qw(@PATH);).
IO constants (SEEK_*, _IO*).
Directory-related IO methods (new, read, close, rewind, tied delete).
INET sockets with multi-homed hosts.
IO poll().
UNIX sockets.
Regression tests for my ($x,@y,%z) : attrs and <sub : attrs>.
File test operators.
Verify operations that access pad objects (lexicals and temporaries).
Verify exists &sub operations.

Incompatible Changes

Perl Source Incompatibilities

Beware that any new warnings that have been added or old ones that have been enhanced are not considered incompatible changes.

Since all new warnings must be explicitly requested via the -w switch or the warnings pragma, it is ultimately the programmer's responsibility to ensure that warnings are enabled judiciously.

CHECK is a new keyword
All subroutine definitions named CHECK are now special. See /"Support for CHECK blocks" for more information.
Treatment of list slices of undef has changed
There is a potential incompatibility in the behavior of list slices that are comprised entirely of undefined values. See /"Behavior of list slices is more consistent".
Format of $English::PERL_VERSION is different

The English module now sets $PERL_VERSION to $^V (a string value) rather than $] (a numeric value). This is a potential incompatibility. Send us a report via perlbug if you are affected by this.

See /"Improved Perl version numbering system" for the reasons for this change.

Literals of the form 1.2.3 parse differently

Previously, numeric literals with more than one dot in them were interpreted as a floating point number concatenated with one or more numbers. Such "numbers" are now parsed as strings composed of the specified ordinals.

For example, print 97.98.99 used to output 97.9899 in earlier versions, but now prints abc.

See /"Support for strings represented as a vector of ordinals".

Possibly changed pseudo-random number generator

Perl programs that depend on reproducing a specific set of pseudo-random numbers may now produce different output due to improvements made to the rand() builtin. You can use sh Configure -Drandfunc=rand to obtain the old behavior.

See /"Better pseudo-random number generator".

Hashing function for hash keys has changed

Even though Perl hashes are not order preserving, the apparently random order encountered when iterating on the contents of a hash is actually determined by the hashing algorithm used. Improvements in the algorithm may yield a random order that is different from that of previous versions, especially when iterating on hashes.

See /"Better worst-case behavior of hashes" for additional information.

undef fails on read only values
Using the undef operator on a readonly value (such as $1) has the same effect as assigning undef to the readonly value--it throws an exception.
Close-on-exec bit may be set on pipe and socket handles

Pipe and socket handles are also now subject to the close-on-exec behavior determined by the special variable $^F.

See /"More consistent close-on-exec behavior".

Writing "$$1" to mean "${$}1" is unsupported

Perl 5.004 deprecated the interpretation of $$1 and similar within interpolated strings to mean $$ . "1", but still allowed it.

In Perl 5.6.0 and later, "$$1" always means "${$1}".

delete(), each(), values() and \(%h)

operate on aliases to values, not copies

delete(), each(), values() and hashes (e.g. \(%h)) in a list context return the actual values in the hash, instead of copies (as they used to in earlier versions). Typical idioms for using these constructs copy the returned values, but this can make a significant difference when creating references to the returned values. Keys in the hash are still returned as copies when iterating on a hash.

See also /"delete(), each(), values() and hash iteration are faster".

vec(EXPR,OFFSET,BITS) enforces powers-of-two BITS
vec() generates a run-time error if the BITS argument is not a valid power-of-two integer.
Text of some diagnostic output has changed
Most references to internal Perl operations in diagnostics have been changed to be more descriptive. This may be an issue for programs that may incorrectly rely on the exact text of diagnostics for proper functioning.
%@ has been removed
The undocumented special variable %@ that used to accumulate "background" errors (such as those that happen in DESTROY()) has been removed, because it could potentially result in memory leaks.
Parenthesized not() behaves like a list operator

The not operator now falls under the "if it looks like a function, it behaves like a function" rule.

As a result, the parenthesized form can be used with grep and map. The following construct used to be a syntax error before, but it works as expected now:

    grep not($_), @things;  

On the other hand, using not with a literal list slice may not work. The following previously allowed construct:

    print not (1,2,3)[0];  

needs to be written with additional parentheses now:

    print not((1,2,3)[0]);  

The behavior remains unaffected when not is not followed by parentheses.

Semantics of bareword prototype (*) have changed

The semantics of the bareword prototype * have changed. Perl 5.005 always coerced simple scalar arguments to a typeglob, which wasn't useful in situations where the subroutine must distinguish between a simple scalar and a typeglob. The new behavior is to not coerce bareword arguments to a typeglob. The value will always be visible as either a simple scalar or as a reference to a typeglob.

See /"More functional bareword prototype (*)".

Semantics of bit operators may have changed on 64-bit platforms

If your platform is either natively 64-bit or if Perl has been configured to used 64-bit integers, i.e., $Config{ivsize} is 8, there may be a potential incompatibility in the behavior of bitwise numeric operators (& | ^ ~ << >>). These operators used to strictly operate on the lower 32 bits of integers in previous versions, but now operate over the entire native integral width. In particular, note that unary ~ will produce different results on platforms that have different $Config{ivsize}. For portability, be sure to mask off the excess bits in the result of unary ~, e.g., ~$x & 0xffffffff.

See /"Bit operators support full native integer width".

More builtins taint their results

As described in /"Improved security features", there may be more sources of taint in a Perl program.

To avoid these new tainting behaviors, you can build Perl with the Configure option -Accflags=-DINCOMPLETE_TAINTS. Beware that the ensuing perl binary may be insecure.

C Source Incompatibilities


Release 5.005 grandfathered old global symbol names by providing preprocessor macros for extension source compatibility. As of release 5.6.0, these preprocessor definitions are not available by default. You need to explicitly compile perl with -DPERL_POLLUTE to get these definitions. For extensions still using the old symbols, this option can be specified via MakeMaker:

    perl Makefile.PL POLLUTE=1  


This new build option provides a set of macros for all API functions such that an implicit interpreter/thread context argument is passed to every API function. As a result of this, something like sv_setsv(foo,bar) amounts to a macro invocation that actually translates to something like Perl_sv_setsv(my_perl,foo,bar). While this is generally expected to not have any significant source compatibility issues, the difference between a macro and a real function call will need to be considered.

This means that there is a source compatibility issue as a result of this if your extensions attempt to use pointers to any of the Perl API functions.

Note that the above issue is not relevant to the default build of Perl, whose interfaces continue to match those of prior versions (but subject to the other options described here).

See perlguts/"The Perl API" for detailed information on the ramifications of building Perl with this option.

    NOTE: PERL_IMPLICIT_CONTEXT is automatically enabled whenever Perl is built
    with one of -Dusethreads, -Dusemultiplicity, or both.  It is not
    intended to be enabled by users at this time.  


Enabling Perl's malloc in release 5.005 and earlier caused the namespace of the system's malloc family of functions to be usurped by the Perl versions, since by default they used the same names. Besides causing problems on platforms that do not allow these functions to be cleanly replaced, this also meant that the system versions could not be called in programs that used Perl's malloc. Previous versions of Perl have allowed this behaviour to be suppressed with the HIDEMYMALLOC and EMBEDMYMALLOC preprocessor definitions.

As of release 5.6.0, Perl's malloc family of functions have default names distinct from the system versions. You need to explicitly compile perl with -DPERL_POLLUTE_MALLOC to get the older behaviour. HIDEMYMALLOC and EMBEDMYMALLOC have no effect, since the behaviour they enabled is now the default.

Note that these functions do not constitute Perl's memory allocation API. See perlguts/"Memory Allocation" for further information about that.

Compatible C Source API Changes


The cpp macros PERL_REVISION, PERL_VERSION, and PERL_SUBVERSION are now available by default from perl.h, and reflect the base revision, patchlevel, and subversion respectively. PERL_REVISION had no prior equivalent, while PERL_VERSION and PERL_SUBVERSION were previously available as PATCHLEVEL and SUBVERSION.

The new names cause less pollution of the cpp namespace and reflect what the numbers have come to stand for in common practice. For compatibility, the old names are still supported when patchlevel.h is explicitly included (as required before), so there is no source incompatibility from the change.

Binary Incompatibilities

In general, the default build of this release is expected to be binary compatible for extensions built with the 5.005 release or its maintenance versions. However, specific platforms may have broken binary compatibility due to changes in the defaults used in hints files. Therefore, please be sure to always check the platform-specific README files for any notes to the contrary.

The usethreads or usemultiplicity builds are not binary compatible with the corresponding builds in 5.005.

On platforms that require an explicit list of exports (AIX, OS/2 and Windows, among others), purely internal symbols such as parser functions and the run time opcodes are not exported by default. Perl 5.005 used to export all functions irrespective of whether they were considered part of the public API or not.

For the full list of public API functions, see perlapi.

Known Problems

Thread test failures

The subtests 19 and 20 of lib/thr5005.t test are known to fail due to fundamental problems in the 5.005 threading implementation. These are not new failures--Perl 5.005_0x has the same bugs, but didn't have these tests.

EBCDIC platforms not supported

In earlier releases of Perl, EBCDIC environments like OS390 (also known as Open Edition MVS) and VM-ESA were supported. Due to changes required by the UTF-8 (Unicode) support, the EBCDIC platforms are not supported in Perl 5.6.0.

In 64-bit HP-UX the lib/io_multihomed test may hang

The lib/io_multihomed test may hang in HP-UX if Perl has been configured to be 64-bit. Because other 64-bit platforms do not hang in this test, HP-UX is suspect. All other tests pass in 64-bit HP-UX. The test attempts to create and connect to "multihomed" sockets (sockets which have multiple IP addresses).

NEXTSTEP 3.3 POSIX test failure

In NEXTSTEP 3.3p2 the implementation of the strftime(3) in the operating system libraries is buggy: the %j format numbers the days of a month starting from zero, which, while being logical to programmers, will cause the subtests 19 to 27 of the lib/posix test may fail.

Tru64 (aka Digital UNIX, aka DEC OSF/1) lib/sdbm test failure with gcc

If compiled with gcc 2.95 the lib/sdbm test will fail (dump core). The cure is to use the vendor cc, it comes with the operating system and produces good code.

UNICOS/mk CC failures during Configure run

In UNICOS/mk the following errors may appear during the Configure run:

	Guessing which symbols your C compiler and preprocessor define...
	CC-20 cc: ERROR File = try.c, Line = 3
	  bad switch yylook 79bad switch yylook 79bad switch yylook 79bad switch yylook 79#ifdef A29K
	4 errors detected in the compilation of "try.c".  

The culprit is the broken awk of UNICOS/mk. The effect is fortunately rather mild: Perl itself is not adversely affected by the error, only the h2ph utility coming with Perl, and that is rather rarely needed these days.

Arrow operator and arrays

When the left argument to the arrow operator -> is an array, or the scalar operator operating on an array, the result of the operation must be considered erroneous. For example:


These expressions will get run-time errors in some future release of Perl.

Experimental features

As discussed above, many features are still experimental. Interfaces and implementation of these features are subject to change, and in extreme cases, even subject to removal in some future release of Perl. These features include the following:

64-bit support
Lvalue subroutines
Weak references
The pseudo-hash data type
The Compiler suite
Internal implementation of file globbing
The DB module
The regular expression code constructs:
(?{ code }) and (??{ code })

Obsolete Diagnostics

Character class syntax [: :] is reserved for future extensions
(W) Within regular expression character classes ([]) the syntax beginning with "[:" and ending with ":]" is reserved for future extensions. If you need to represent those character sequences inside a regular expression character class, just quote the square brackets with the backslash: "\[:" and ":\]".
Ill-formed logical name |%s| in prime_env_iter
(W) A warning peculiar to VMS. A logical name was encountered when preparing to iterate over %ENV which violates the syntactic rules governing logical names. Because it cannot be translated normally, it is skipped, and will not appear in %ENV. This may be a benign occurrence, as some software packages might directly modify logical name tables and introduce nonstandard names, or it may indicate that a logical name table has been corrupted.
In string, @%s now must be written as \@%s

The description of this error used to say:

        (Someday it will simply assume that an unbackslashed @
         interpolates an array.)  

That day has come, and this fatal error has been removed. It has been replaced by a non-fatal warning instead. See /Arrays now always interpolate into double-quoted strings for details.

Probable precedence problem on %s

(W) The compiler found a bareword where it expected a conditional, which often indicates that an || or && was parsed as part of the last argument of the previous construct, for example:

    open FOO || die;  

regexp too big
(F) The current implementation of regular expressions uses shorts as address offsets within a string. Unfortunately this means that if the regular expression compiles to longer than 32767, it'll blow up. Usually when you want a regular expression this big, there is a better way to do it with multiple statements. See perlre.
Use of "$$<digit>" to mean "${$}<digit>" is deprecated

(D) Perl versions before 5.004 misinterpreted any type marker followed by "$" and a digit. For example, "$$0" was incorrectly taken to mean "${$}0" instead of "${$0}". This bug is (mostly) fixed in Perl 5.004.

However, the developers of Perl 5.004 could not fix this bug completely, because at least two widely-used modules depend on the old meaning of "$$0" in a string. So Perl 5.004 still interprets "$$<digit>" in the old (broken) way inside strings; but it generates this message as a warning. And in Perl 5.005, this special treatment will cease.

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. 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.


The Changes file for 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.


Written by Gurusamy Sarathy <>, with many contributions from The Perl Porters.

Send omissions or corrections to <>.




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