I’ve been starting to make use of the new features introduced in perl 5.10 recently (after being constrained by my main dev environments still running perl 5.8.8, and not having the time to upgrade).

My favourite features so far are:

The smart match operator

The new smart-match operator, ~~, is a great example of DWIM.

A few examples:

if (@a ~~ 'foo')  # list contains at least one item equalling 'foo'
if (@a ~~ /foo/) # list contains at least one item matching /fo+/
if (@a ~~ @b)   # lists contain same values

That’s just a brief overview; there’s plenty more documentation

say

Not a big change, but the new say keyword acts just like print, but adds an implicit newline to the end – so say 'Hello'; is just the same as print "Hello\n";

It’s more useful in cases where you would have had to add parenthesis to get correct precedence – something like: print join(';', @foo) . "\n"; can now be written more concisely as just say join ';', @foo;.

Switch (given) statement

given ($foo) {
    when (/^abc/) { abc(); }
    when (/^def/) { def(); }
    when (/^xyz/) { xyz(); }
    default { die "Unrecognised foo"; }
}

Defined-or

// is now the defined-or operator.

It’s pretty common to use conditional assignments like: $a ||= $b to assign to $a unless $a already has a value. Now you can use $a //= $b to test for definedness rather than truthiness.

Likewise, if ($hash{foo} // $hash{bar}) will be true if either of them is defined (even if they’re defined but have a false value).

Named regex captures

Parenthesised sub-expressions in regular expressions can now be given a name, and accessed via the special %+ hash:

if ($foo =~ m{ (? \d{4} ) - (? \d{2}) - (? \d{2}) }xms) {
    say "Year: $+{year}";
}

The features above are my own personal favourites, in no particular order. The full (large) set of changes can be found in the perldelta for 5.10.0.