If i really understand your need :

$ head -n3 data.test
--header--
sdfsdfsdfs
sdfsdfsdfs

$ ls -lh data.test
-rw-r--r-- 1 user user 525M 2008-07-18 11:11 data.test

# Linux debian 2.6.18-xen #1 SMP x86_64 GNU/Linux

$ time perl -i -pe '1 .. s/^.*//ms' data.test

real    0m48.445s
user    0m35.550s
sys    0m2.760s

$ head -n3 data.test
sdfsdfsdfs
sdfsdfsdfs
sdfsdfsdfs
[download]

hth,
PooLpi