Changes since Perl 5.14

According to my notes, the last time I spoke about Perl updates, the current
version was 5.14, which was new and shiny in 2011.

Just a month ago, Perl 5 turned 20, so it's no longer a teenager. A language
doesn't endure that long without having been basically sound from the start,
so it's hardly surprising that developments now are rarely earth-shattering.

A lot of changes in the recent versions are devoted to ensuring that new and
improved features can be incorporated without breaking old code. A good deal
of the updates are to the core modules, rather than the language itself.

One of the problems I found while preparing this talk was the extreme obscurity
of many of the changes, and consequent difficulty of even explaining them to
a general audience, let alone making them interesting. In many cases, it seems
that if you understand them, it was probably your bug report that initiated
them in the first place. However, I'll accept the challenge, but I'll skip
some.

May 2012 brought 5.16.0. Judging by the very modest changes in the 3 point
releases, the update was fairly trouble-free.

Changes:

use VERSION, where VERSION is something like v5.16 or 5.016, now clears all
the old features before enabling the new set:

    use 5.016;
    # only 5.16 features enabled here
    use 5.014;
    # only 5.14 features enabled here (not 5.16)

Which would be a pretty strange thing to do like that, but might be needed
to protect some old code that would otherwise break.
Anything 5.12 or later enables "use strict" by default but explicit use strict 
and no strict now override the version declaration, even when they come first:

    no strict;
    use 5.012;
    # no strict here

You can do even stranger things with "no feature", but I don't propose to 
encourage that sort of behaviour.

Something else that should be discouraged; $[ is now disabled under use v5.16. 
It can be turned on or off explicitly with use feature 'array_base' .

Likely one of the biggest features of 5.16 is current_sub.

If you "use v5.16" or "use feature 'current_sub' __SUB__ returns a reference 
to the current subroutine, making it easier to write recursive closures.
Probably not a bad idea even in an ordinary recursive subroutine, because the
behaviour will not change, even if you rename the routine.

    use 5.16.0;
    my $factorial = sub {
    my ($x) = @_;
    return 1 if $x == 1;
    return($x * __SUB__->( $x - 1 ) );
    };

    say $factorial->(3);

eval not knowing whether it's dealing with bytes or unicode
can be disambiguated with unicode_eval and evalbytes features that force eval
to work one way or the other. Guess how?

substr used as an lvalue has had a change made to its behaviour. If you've done
that, you might want to check the delta to see if it affects you.

tied now returns a value that lets you weaken the tie.

Unicode support seems to be one of the bigger features of recent changes. We're
not in ASCII any more, Toto, but it's hard to be definitive about features the
Unicode people haven't completely sorted out themselves.  Characters have been
renamed, as documented in the perldelta. Perl's tracking the changes, but there
are invitable delays. One is particularly noteworthy; since the dawn of time, 
ASCII 007 has been BELL, but now that relates to some feature of Japanese 
cellphones, and ALERT is what will ding the console bell. See charnames in
perldoc for details.

Unicode characters can also be used in Perl code; I hope I don't have to work
with any that does that, unless I have the same keyboard. 

use locale has acquired some extra features, and there's a "foldcase" feature
fc to enable comparison of Unicode strings. XS also has some new features
relating to Utf-8.

A couple of small changes to special variables; $$ is now writable, though I
have difficulty understanding why you'd want to do that, and $^X is now an
absolute path on some of the BSD variants. (Perl executable's name.)

There are some changes to the debugger, which you'll probably need if you 
start messing around with Perl's CORE:: functions; hackers only?

There are a lot of upgrades to modules and documentation, along with future 
deprecations and warnings about build features for really old OSs. At least
half of the delta deals with features in the murky depths of Perl's internals
that shouldn't concern anyone but Perl porters. If you're interested in that
stuff you're probably smarter than I am.