Perhaps...

#!/usr/bin/perl -w
use strict;

my $fileName = "abc";

open(IN, '<', "$fileName") or die "Could not open file $fileName\n";
open(OUT, '>', "$fileName.tmp") or 
            die "Could not open file $fileName.tmp\n";
while (<IN>) 
{
    s/foo/bar/g;
    print OUT $_;
}
close IN;
close OUT;
unlink   $fileName;
rename  "$fileName.tmp", $fileName;
[download]