Three possibilities off the top of my head.

First, do your substitutions on the string returned from as_HTML. A little error-prone, but probably not that bad.

Second, stop using HTML::Element. Use HTML::Template instead. You could put the template in your __DATA__ section.

Third, use Template Toolkit - instead of having WWW and HHH, etc., have [%WWW%] and [%HHH%], etc. And then you can feed the as_HTML output into Template Toolkit to do the next level of modification. You can't easily do this with HTML::Template only because having <tmpl_var name="HHH"> instead of just HHH would get modified by HTML::Element to look like &lt;tmpl_var name="HHH"&gt; inside the tags, and that would defeat it. TT2 could handle that largely because it uses different delimiters.

I realise that you are probably looking for a way to traverse the data structure. That's just not the way I'm looking at the problem - I have a different box to think inside of ;-}