Apache::SSI implements the functionality of mod_include for handling
server-parsed html documents. It runs under Apache's mod_perl. In my mind,
there are two main reasons you might want to use this module: you can
sub-class it to implement your own custom SSI directives, and/or you can
parse the output of other mod_perl handlers, or send the SSI output through
another handler (use Apache::Filter to do this).
