So suddenly my problem is reduced to including variable files! Unfortunately a similar lack of recusive support means this doesn't work: <!-- tmpl_include name='<!-- tmpl_var name='filename' -->'>

try HTML::Template::Compiled*) - it offers TMPL_INCLUDE_VAR which includes the template file given (it also offers TMPL_INCLUDE_STRING, so that you can pass a template string into another template, but that might cost a bit of performance)
*) disclaimer: i'm the author.