Compare first the years and then, if the years are equal, look at the months. If the months are the same, look at the days. This gives rise to code like this:
sub cmp_by_bits {
my( $ad,$am,$ay,$bd,$bm,$by );
$_[0] =~ /(\d+)\/(\d+)\/(\d+)/ # dd/mm/yy{yy}...