That funny syntax represents the index of the last element of an array starting from 0, eg:

#!/usr/bin/perl

use warnings;
use strict;

my @a = qw(0 1 2 3 4 5);

print "$#a\n\n";

for my $i (0 .. $#a){
    print "$i\n";
}

print "elem 5: $a[$#a]\n"; # same as $a[-1] or $a[5]
[download]

Output:

5

0
1
2
3
4
5
elem 5: 5
[download]

-stevieb