Indeed, that nails it. Once a better benchmark is used, so that the lvalue variant no longer shows as slower than both the reverse-chop-reverse and regex versions, then the much smaller difference between lvalue and 4-arg makes much more sense, in the light of this interpretation of the opcode tree. Thanks.

It's always good to air these things here rather than bothering p5p with them. And who knows, maybe the simple case optimisation will turn up at some point in the future. The map in a void context probably wasn't a item on anyones high priority list either, but someone found the problem interesting enough to pursue it.