As you've already said, they probably won't like a flood of automated requests, so it's best to see if the site has some terms of service that you should follow so that they don't block you or worse.

Keeping that in mind, it sounds like WWW::Mechanize might be able to help you - or, for a fixed URL, plain old LWP::Simple should work, since it sounds like basically what you want to do is refresh the page multiple times and then compare the versions. Once you've got the HTML, something like HTML::FormatText could give you the text-only version, which could be diffed using something like Text::Diff (disclaimer: I haven't used these latter two modules, just found them via a quick search). Or, you could get the structure of the HTML via something like HTML::TreeBuilder and implement the extraction and comparison yourself.