ajt's right. You really do want to use XML tools for processing XML. The only time you may possibly do better with regexes is when you're writing a one-off script that only parses a very regular short file that you've inspected before running the script, and it generally takes a couple of tries even to get that right.

In other circumstances just the fact that a real XML parser will throw a huge tantrum on invalid input will already safe you a lot of work. And that's without mentioning some of the really nice interfaces that modules like XML::Twig can provide.