aside from the off-by-one issue already mentioned, if there are still undef's you can do:

   my $sum = 0;
   $sum += ($weights[$_]||0) * ($digits[$_]||0) for 0 .. @#weights;
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Also note that, in general, $x = $x || 0; can be stated as $x ||= 0; Another way:

   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;
[download]