my $sum = 0;
   $sum += ($weights[$_]||0) * ($digits[$_]||0) for 0 .. @#weights;

##</code><code>##

   my $sum = 0;
   for (0..@#weights) {
      next unless $weights[$_] && $digits[$_];
      $sum += $weights[$_] * $digits[$_];
   }
# or
   for (0..@#weights) {
      $sum += $weights[$_] * $digits[$_] if $weights[$_] && $digits[$_];
   }
# or yet another:
   $sum += $weights[$_] * $digits[$_] for grep $weights[$_] && $digits[$_], 0..@#weights;