OK, I'll have a go at my own question then :)

The reason for this is the HTML templates were rather 'pretty' and had a heap of nested tables in them that was just too much code to duplicate. So I wanted a way to call a function from within the template that can accept a parameter and produce the code for the table with the parameter as the title of the table.

In the end I found that HTML::Template::Expr can do this so I modified CGI::Application to use HTML::Template::Expr instead of HTML::Template and it all went well. It was just a simple substitution of the module name in two spots.

I know this is not a good use of templating technology, both I and the HTML::Template author agree there - but this particular case called for it.

The actual code in the CGI::Application module was a declaration of the template functions and their related subs:

HTML::Template::Expr->register_function(box => \&box);

sub box {
   #read in other templates to costruct table header
   #use parameters to decide if it is the start or the
   #end of the box and what title to use etc.
}
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Then in the templates:

<!-- TMPL_VAR expr="box('box-start','Table title')" -->
  table contents here
<!-- TMPL_VAR expr="box('box-end')" -->
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Ugly, but it works and reduces the template code size to about half :)