use strict;
...
sub compare {
$a =~ /(\d{2})\/(\d{2})\/(\d{4})/;
$c = $3 . $1 . $2;

$b =~ /(\d{2})\/(\d{2})\/(\d{4})/;
$c = $3 . $1 . $2;

$c <=> $d;
}