I skipped the bit where you set up the names of input file and output file. But generally speaking, you should always always 'use strict;' and 'use warnings;'. They really are the very best ways to stop a program doing anything weird.
And as mentioned above- a while look is better than a foreach if you're processing a large file. (Makes little odds for a small file, but it's good form).
Perl is very clever - it understand context. <$input_fh> says 'read from $input_fh' but if you do:
It will read the whole file into that array - which is in effect what my first snippet does. It doesn't make much difference if you're working with a small file, but the difference will become very important with a 500MB file.
I'd strongly suggest taking time to understand what each line is doing - code that someone on the internet gave you is never trustworthy. (Although on Perlmonks, usually true evil will get stomped upon)