while(<>) {
   ($a, $b, $c, $d) =~ /(...)(...)(...)(...)/;
   $hash{$a}{$b}{$c}{$d}++;
}

##</code><code>##

 foreach $key (keys %hash) {
   foreach $key1 (keys %{$hash{$key}}) {
      foreach $key2 (keys %{$hash{$key}{$key1}}) {
      # do something
      }
    }
  }