I wrote a module I call 'Text::SSITemplate' to get around the fact that mod_include (or whatever you use to parse your SSIs) never sees the output of a CGI program. This is mostly an implementation of Apache's mod_include without 'exec':

SSITemplates.tar.gz

The tarball includes pod documentation, an example cgi, example templates, and the SSITemplate library. I'd be really interested to hear what folks think of this (and whether I should put it on the CPAN).

This does not require mod_perl, but if you run mod_perl there is an Apache::SSI on the CPAN that you may want to look at.