No, it doesn't hurt performance. And no, Perl does not have a JIT ("just in time") compiler. Perl has an "on startup" compiler, it goes through a a compile phase first when your script is first loaded. Actually, it goes through the source (or its parsed equivalent, the parse tree) a few times, executing various types of special code blocks: BEGIN, INIT, CHECK, at the various passes. The comments are thrown out when the source is first parsed. So it might slow down parsing for a few milliseconds — most of it you can blame on the time to read the source from disk.