The task that started me on the quest for speed was indeed a monster regex, it was also being called many times in a tight loop. As the regex was essentially repetitious, I originally thought that compiling the base regex with qr// and then using that in conjuction with a repeat count and the none repeating elements also compliled with /o might leach some benefits, but the reverse was true. I settled for programically generating the regex (using x n) into a single large regex and then compiling it with qr// (which appeared to give some slight performance benefit over /o). This was possibly due to the fact that when compiled with qr//, you can use the resultant variable directly ($string =~ $compiled_re) rather than needing to embed it within an m// operator (m/$compiled_re/). Maybe its slightly quicker to execute the former than the latter? The difference seemed significant enough to make it worthwhile..