#!/usr/bin/perl
$fileName = "abc";

open(IN, "<", "$fileName") or die "Could not open file $fileName";
read IN, my $new, -s IN,
close (IN);

$new =~ s/foo/bar/g;

# Open the the file in write mode
open(OUT, ">", "$fileName") or die "Could not open file $fileName";
# print the new contents
print OUT $new;
close (OUT);