The @h{@a,@b,@c,@d} syntax is a hash slice and it returns a list of all values in %hash that corresponds to keys represented by the array elements. It will auto-vivify elements that do not exist (hence your "uninitiliazed value" warnings). The preincrement operator is apparently taking the final element of the list and incrementing it. In other words, this is an obfuscated way of incrementing a single hash value, but with the side effect of auto-vivifying others. The following code snippet demonstrates:

use strict;
use warnings;
use Data::Dumper;
$Data::Dumper::Indent = 1;

my %hash = (
    foo  => 1,
    bar  => 2,
    baz  => 3,
    quux => 4
);
my @a = qw(foo bar asdf);
my @b = qw(baz);
++@hash{@a,@b};

print Dumper \%hash;
[download]

That auto-vivifies $h{asdf} and increments $h{baz} to 4.

Cheers,
Ovid

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