sub by_date {
  my ($monA, $monB, $dayA, $dayB, $yearA, $yearB);
  ($monA, $dayA, $yearA) = split(/\//, $a);
  ($monB, $dayB, $yearB) = split(/\//, $b);
  $yearA <=> $yearB
         or
  $monA <=> $monB
         or
  $dayA <=> $dayB
}

foreach $k (sort by_date %H) {
...
}