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

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