> The '/i' doesn't override how the pattern has already been compiled.

Oh, I see. You're right Dave, my bad.

> Note also that case-insensitive matching is always going to be much slower than case-sensitive matching, especially when UNICODE is involved.

It really is awfully slow:

$ RE=... # an actual regex here
$ time fbgrep.pl "$RE" *fb2* >/dev/null
real    0m32.912s
user    0m32.374s
sys     0m0.483s

$ time fbgrep.pl -i "$RE" *fb2* >/dev/null
real    2m17.575s
user    2m16.359s
sys     0m0.421s
[download]

But I suppose there's not much to be done about it.

Thanks for pointing me in the right direction!