Lots of information about that subject can be found here. The brief answer is that you can get Apache to parse .html files, but you need to do a few things (chmod and edit your httpd.conf). Your question seems to be about the file name itself, as opposed to Perl issues. If I'm wrong, Masem posts good advice above.

Update: ichimunki pointed out to me that you were actually trying to include an SSI in Perl output. <notetoself>Read the Fine Code</notetoself> ichi's suggestions below are what I would recommend, personally, but as I stated in the original post, Masem notes CGI::SSI will help you do what you originally planned.