True, one regex is faster than a hash of regexes... certainly any case that can be accomplished with one shouldn't be squeezed into a data structure. But, I think it is bad to have a hash with closures as values where the closures are repeated and required to be the same... better might be to stick the regexes in an array in that case, and just use $count++ or a regular function instead of anonymous functions. Once you get it distilled that way, to whatever simplicity your problem domain allows, I suspect you'll almost always be able to pick one or the other based on simplicity...

OTOH, a 6 mile regex could be tough to debug... what would be nice would be a module that would let you put in regexes, and tell it to do the matching one way or the other, that way if you've got a lot of them you can switch to the looping mode when you need to find the broken bits.
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