You can't do it within HTML::Template, period. You can do it within Template Toolkit simply because TT allows you to create recursive functions which are the easiest way to handle recursive data structures, such as trees.
Furthermore, is there a reason you're using a flat array vs. one of the CPAN solutions like Tree or Tree::Simple? Tree::Simple provides for a huge number of visitors, some of which handle layout in terms of spaces. Tree has an persistence mode that writes to an XML file that you can use XSL to transform into HTML. Heck, it would be extremely simple to write a "persistence" layer for Tree that persists to HTML vs. XML and writes to an IO::Scalar object vs. a filehandle.
My criteria for good software:
- Does it work?
- Can someone else come in, make a change, and be reasonably certain no bugs were introduced?