$ 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