I specifically avoided undef @h{}; for being cryptic.

That's at least part of the reason I used it ;). It's a weird idiom, but useful when you want to use hashes as sets rather than maps, so I think more people should be introduced to it. Besides, it creates fewer scalars and executes fewer operations than other methods, and the OP did say "fastest". That said, I like your solution (to a slightly different problem) -- simple, terse, and no bizarro modules.