If you are using mod_perl you can try Apache::Filter+Apache::SSI or Apache::SSIChain (based on Apache::OutputChain and Apache::SSI). The second variant permits you to filter almost all - plain HTML, output from other mod_perl scripts or from usual CGI scripts written in any language, and so on.

Both variants are fully mod_include complaint and have two additional powerful features (well, Apache::SSI have them):

• calling perl subs and including results
• really good if..elif..else construct)