#!/usr/bin/perl

use strict;
use warnings; # or just #!/usr/bin/perl -w on first line

my @dates = ('20130401', '20130501', '20130601');
my @ordered = sort { &compare } @dates;
print "@ordered\n";

sub compare {
    $a =~ /(\d{4})(\d{2})(\d{2})/;
    my $c = $3 . $2 . $1;
    $b =~ /(\d{4})(\d{2})(\d{2})/;
    my $d = $3 . $2 . $1;
    $c <=> $d;
}