find and replace in every file under a root folder

From: Cortes (tvhoang1980_at_yahoo.com)
Date: 06/07/04 

Date: Tue, 8 Jun 2004 00:02:30 +0800

Suppose I want to add 1 single line:

<script src = "/sample.js" type="text/javascript"></script>

to every single webpage ( preferrably right after the tag <head>) under the
root folder called public_html in my server, i.e including all the webpages
in the subfolders as well. Suppose I have sample.js in public_html as well.

Can someone please spend some of your precious time to help me out with this
script because I know nothing about Perl but I really need the script. I've
done a search but no result.
Thanks very much in advance