Unfortunately there are much deeper problems as far as dynamic includes are concerned. You could go the way of including a filter, however, this won't work the way you expect it to if you cache the parsed template (the filter doesn't get processed on cached or file_cached templates). The most common solution to this problem is to go ahead and process one of the inner templates and include it as a parameter to the outer. If you've been following the html-template mailing list recently, there's been a good amount of talk concerning changes Sam Tregar hopes to implement in version 3.0. It seems that the changes would allow templates to be included dynamically if not directly through an included method then at least by changing the template's parse structure itself.
Of course, I know of a couple of projects currently being worked upon to implement some new features on top of HTML::Template, however, I'm unable to recall any of them at this moment.