400 alternations is manageable, but don’t forget the match is executed 50,000 times.

Regexp::Assemble helps by reducing the necessary backtracking, but don’t forget that Regexp::Assemble itself doesn’t work by magic and will need to do a fair amount of string munging. Most likely, that effort is easily amortised during the actual tests, but you don’t know that yet. Then the question that remains is how similar the lines are; if they all differ greatly, then Regexp::Assemble can’t do much.

Without benchmarks I wouldn’t go on the record with any prediction as to what’s going to be faster.

And the problem that the code does not match intention closely remains. The hash-based approach will be trivial to get right, and it will be much easier to understand in six months.

Makeshifts last the longest.