Thanks, that's a great explanation!! I hadn't looked at the code at all, just the changelog itself, so it didn't make a lot of sense except that I had assumed the methods must have been mostly or solely "getters" or it wouldn't have worked.

I didn't realise dereferencing $u like that was actually optimisation too - I assumed that Perl would have optimised for that. Presumably it doesn't optimise because - at least in theory - one of the things done inside the loop might have changed $u so Perl has to check?

Thanks for these posts - they're really helpful.