While I'm not certain about this, I seem to recall reading that loading POSIX is pretty cheap if you limit yourself to the imports you need. Or is that hogwash?

At any rate, I'd just about always prefer Time::Local for the mentioned reasons.

More computing sins are committed in the name of efficiency (without necessarily achieving it) than for any other single reason - including blind stupidity.
-- William A. Wulf, A Case Against the GOTO

Makeshifts last the longest.