s/x/"o" x 5/e is rather inefficient, specially if you do it in a loop. It's inefficient because you ask Perl to execute something that always gives the same result. s/x/ooooo/ or $o = "o" x 4; ...; s/x/$o/ are more efficient.

s///e is interesting if the result of the code in the replacement part depends on the match. s/(\d+)\s*\+\s*(\d+)/$1 + $2/e; for instance.

Abigail