This is probably more of an Apache question than a Perl one. From my reading of the documentation, it looks like
PATH_INFO should be available. The
mod_include documentation says that all standard CGI variables should be available to SSI documents, the
mod_cgi documentation says that all variables in the
CGI specification will be set, and the
CGI Environment Variables part of the spec includes
PATH_INFO. So I'm not sure why you're seeing the problem you're seeing, but an Apache guru would probably know.