Split the DATA section into 2 parts? I think that's better done using here-docs. Granted, the strings will be intertwined in between the chunks of Perl code instead of at the end, but that doesn't have to be a disadvantage; especially with lots and lots of html snippets. I think having to count in order to know what string you're using where is a bad situation as soon as it is no longer trivial.

And perhaps you should reconsider using a templating system, for example, Template Toolkit. At least the html data will be organized by the page layout, and not by the code flow — which can be like spaghetti code, if you want to follow how a page is built.