Don't. At least for Amazon.com. They have a fairly deep API into the stuff (you have to sign up for a dev token). This is new and nice -- URI::Amazon::APA; and there are a couple of other things on the CPAN to get into there.

You get back XML for different search types and do whatever you want with it; info including used items, lowest prices, related items, reviews, ratings, whatever. It's not super easy (because there are so many options) but they have pretty good docs and it will always be more robust that HTML parsing. They even have an XSLT service too so you can even get your stuff back preformatted.