Hmm. This sounds similar to the problem we have with browser-detection as opposed to feature-detection. Wouldn't it be better, instead of saying "I need version 5.20 semantics", to say "I need X semantics", where X is the name of a semantic set. Implementing this feature at this late stage in perl's development would probably require significant work, but would avoid module/script authors having to determine which semantics changed in which versions. If you care about them, you ask for what you want.