Well, it will have an impact, but mostly if it actually triggers a warning/error - because then it does work.
But it will only do extra work if there's a problem, and further more, if such tiny resource uses are important for the overall running time of your program, you shouldn't have used Perl in the first place.
It's like after having choosen the Winnebago over the Ferrari, you wonder whether driving with side mirrors impacts its performance. (The answer to that is "Yes, side mirrors impact performance, but who cares about the difference, and wouldn't it be much better to leave them on in the first place?")
