in reply to most efficient way to implement "A without B"
It might be worthwhile to take a look at the List:: modules on CPAN, specifically List::Util and List::Compare. List::Util provides functions like min, max, and sum. List::Compare provides intersection, union, and (of relevance to the posted question) ways to determine which elements appear in one list but not the other.
Zaxo provided a very nice (and probably more efficient) solution, but the List:: modules will come in handy if you have to do more complicated functions.
HTH
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