Oops, missed the undef.

But note that starting at one has nothing to do with the dynamic programming technique. If you wanted to start at zero (say if @S and @T are inputs to the function), then just replace

               $L[$i-1][$j-1] ||= 0;
               $L[$i][$j] = $L[$i-1][$j-1] + 1;
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with

               if ($i && $j) {
                  $L[$i-1][$j-1] ||= 0;
                  $L[$i][$j] = $L[$i-1][$j-1] + 1;
               } else {
                  $L[$i][$j] = 1;
               }
[download]