Since there is a subroutine call there is some overhead in making the change. However, the subroutine itself only changes some bits in $^H, so it's not a difficult call. If you can wrap the no strict 'refs' around a larger lexical scope you may benefit (such as around a loop). Perhaps you can make sure you don't use anything but hard refs except for that part, and keep it strict 'refs' on until you finally ship it the script. It'll probably be strict happy, but there is some risk involved. I wouldn't worry much though.
Taking the strict / no strict up some loop level(s) is what i've done in the past, and since my current project is in-house only, i think i'll do the same, in that while debugging, i'll keep strict in the innermost loop, then pull it up some level(s) for speed.
thanks, all