I don't think you can without serious acrobatics, if at all; Apache (which I assume you're using) allows you to define different handlers for certain kinds of files, and while you can chain handlers together under mod_perl, the logistics are bound to be incredibly hairy.

Let's push the question back a stage. PHP is very powerful, almost as powerful as Perl; it is certainly far more powerful than server-side includes. What is it that you can't do with pure PHP that you would want to do in Perl SSI (or,catering to the present audience, vice-versa) ?

Philosophy can be made out of anything. Or less -- Jerry A. Fodor