=head2 Interpreter cloning, threads, and concurrency
-Perl 5.005_63 introduces the beginnings of support for running multiple
+Perl 5.6.0 introduces the beginnings of support for running multiple
interpreters concurrently in different threads. In conjunction with
the perl_clone() API call, which can be used to selectively duplicate
the state of any given interpreter, it is possible to compile a
NOTE: This is currently an experimental feature. Interfaces and
implementation are subject to change.
-=item Support for CHECK blocks
+=head2 Support for CHECK blocks
In addition to C<BEGIN>, C<INIT>, C<END>, C<DESTROY> and C<AUTOLOAD>,
subroutines named C<CHECK> are now special. These are queued up during
For example to match alphabetic characters use /[[:alpha:]]/.
See L<perlre> for details.
-=item Better pseudo-random number generator
+=head2 Better pseudo-random number generator
In 5.005_0x and earlier, perl's rand() function used the C library
rand(3) function. As of 5.005_52, Configure tests for drand48(),
now correctly prints "3|a", instead of "2|a".
-=item Better worst-case behavior of hashes
+=head2 Better worst-case behavior of hashes
Small changes in the hashing algorithm have been implemented in
order to improve the distribution of lower order bits in the
=head2 Boolean assignment operators are legal lvalues
-Constructs such as C<($a ||= 2) += 1> are now allowed.
+Constructs such as C<($x ||= 2) += 1> are now allowed.
=head2 exists() is supported on subroutine names
is pointing at, instead of always going to the underlying C runtime
library's C<stderr>.
-=item More consistent close-on-exec behavior
+=head2 More consistent close-on-exec behavior
On systems that support a close-on-exec flag on filehandles, the
flag is now set for any handles created by pipe(), socketpair(),
because other untrusted processes can modify messages and shared memory
segments for their own nefarious purposes.
-=item More functional bareword prototype (*)
+=head2 More functional bareword prototype (*)
Bareword prototypes have been rationalized to enable them to be used
to override builtins that accept barewords and interpret them in
This behavior must be specifically enabled when running Configure.
See F<INSTALL> and F<README.Y2K>.
+=head2 Arrays now always interpolate into double-quoted strings
+
+In double-quoted strings, arrays now interpolate, no matter what. The
+behavior in earlier versions of perl 5 was that arrays would interpolate
+into strings if the array had been mentioned before the string was
+compiled, and otherwise Perl would raise a fatal compile-time error.
+In versions 5.000 through 5.003, the error was
+
+ Literal @example now requires backslash
+
+In versions 5.004_01 through 5.6.0, the error was
+
+ In string, @example now must be written as \@example
+
+The idea here was to get people into the habit of writing
+C<"fred\@example.com"> when they wanted a literal C<@> sign, just as
+they have always written C<"Give me back my \$5"> when they wanted a
+literal C<$> sign.
+
+Starting with 5.6.1, when Perl now sees an C<@> sign in a
+double-quoted string, it I<always> attempts to interpolate an array,
+regardless of whether or not the array has been used or declared
+already. The fatal error has been downgraded to an optional warning:
+
+ Possible unintended interpolation of @example in string
+
+This warns you that C<"fred@example.com"> is going to turn into
+C<fred.com> if you don't backslash the C<@>.
+
+See L<http://www.plover.com/~mjd/perl/at-error.html> for more details
+about the history here.
+
=head1 Modules and Pragmata
=head2 Modules
Minor changes in how subroutine calls are handled internally
provide marginal improvements in performance.
-=item delete(), each(), values() and hash iteration are faster
+=head2 delete(), each(), values() and hash iteration are faster
The hash values returned by delete(), each(), values() and hashes in a
list context are the actual values in the hash, instead of copies.
Remember that "my", "our", and "local" bind tighter than comma.
+=item Possible unintended interpolation of %s in string
+
+(W ambiguous) It used to be that Perl would try to guess whether you
+wanted an array interpolated or a literal @. It no longer does this;
+arrays are now I<always> interpolated into strings. This means that
+if you try something like:
+
+ print "fred@example.com";
+
+and the array C<@example> doesn't exist, Perl is going to print
+C<fred.com>, which is probably not what you wanted. To get a literal
+C<@> sign in a string, put a backslash before it, just as you would
+to get a literal C<$> sign.
+
=item Possible Y2K bug: %s
(W y2k) You are concatenating the number 19 with another number, which
that are comprised entirely of undefined values.
See L</"Behavior of list slices is more consistent">.
-=head2 Format of $English::PERL_VERSION is different
+=item Format of $English::PERL_VERSION is different
The English module now sets $PERL_VERSION to $^V (a string value) rather
than C<$]> (a numeric value). This is a potential incompatibility.
See L</"More functional bareword prototype (*)">.
-=head2 Semantics of bit operators may have changed on 64-bit platforms
+=item 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,
See L</"Bit operators support full native integer width">.
-=head2 More builtins taint their results
+=item More builtins taint their results
As described in L</"Improved security features">, there may be more
sources of taint in a Perl program.
These expressions will get run-time errors in some future release of
Perl.
-=head2 Windows 2000
-
-Windows 2000 is known to fail test 22 in lib/open3.t (cause unknown at
-this time). That test passes under Windows NT.
-
=head2 Experimental features
As discussed above, many features are still experimental. Interfaces and
might directly modify logical name tables and introduce nonstandard names,
or it may indicate that a logical name table has been corrupted.
+=item 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 L</Arrays now always interpolate into double-quoted strings> for
+details.
+
=item Probable precedence problem on %s
(W) The compiler found a bareword where it expected a conditional,
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