$ echo -e "old\nfarts\nsmell"  > foo
$ cat foo
old
farts
smell
$ perl
use strict;
use warnings;
my $fh = undef;
open ($fh, "<", 'foo') or die "Can't open: $!";
while (<$fh>) {
	chomp;
	$fh =~ s/old/new/g;
}
close ($fh) or die "Can't close: $!";
__END__
$ cat foo
old
farts
smell