Quantum::Superpositions - Takes a whole class of simple operations that were too complex in Perl and makes them simple again (comparing lists, set operations, etc) - that was a suprise for me. The name does not idicate that it's such a general purpose module.
Quantum::Superpositions for comparing lists, set operations, etc
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I've known about Q::S for a long time, but I always thought it's a joke (and inefficiently implemented) that TheDamian forgot to put inside the Acme namespace :) (Don't know where I got that impression). However, I happily admit that I was wrong -- there is quite a number of articles and nodes about Q::S now that show its usefulness. It was only today that Randal posted a link showing how to win football bets all the time using Q::S :).
Also, recent versions seem to be rather efficient. I haven't looked at the source yet, but I assume that it's more efficient and better debugged than anything I'd quickly hack together if I were in the need for it.