rename $filename, $filename.".bak" or die "$!";
open(IN, $filename.".bak") or die "$!";
open(OUT, ">".$filename) or die "$!";

while (<IN>) {
     s/content/newcontent/g;
     print OUT;
}
close OUT;
close IN;

##</code><code>##

open(IN, $filename) or die "$!";
open(OUT, ">".$filename.".new") or die "$!";

while (<IN>) {
     s/content/newcontent/g;
     print OUT;
}
close OUT;
close IN;
rename $filename.".new", $filename or die "$!";