Perl help needed

I could use a spot of help from somebody who knows their way around Perl. The task at hand is to write a reliable script that can do a multi-line replacement across multiple files in multiple subdirectories -- to take X (longish) chunk of text and swap Y chunk into its place.

If you've got the skills to do that, and the free time to do it in the next day or so, please drop me an email at marie -dot- brennan -at- gmail -dot- com.