@a = (['a',1],['a',2],['b',3],['b',4]);
   $last = ['', ''];

   for $i (0..$#a) {

      $this = $a[$i];
      $next = defined $a[$i+1] ? $a[$i+1] : $last;

      $total += $this->[1];

      if ($this->[0] ne $next->[0]) {
         print "$this->[0]: $total\n";
         $total = 0;
      }
   }