But it seems to be that with modperl as DSO this directive does not work, and that Apache::include is only a modperl1.x funcionality, so discarted in mod perl 2.
I've started this port, but it's a long way from being completed - it doesn't work right and dumps core, but I haven't looked at it in quite a while. oh, and it's really ugly too. in other words, very much a work in progress.
here it is if you want to help - I'll probably get back to it in a few weeks.
http://www.modperlcookbook.org/~geoff/modules/experimental/Apache-SSI-2.0.tar.gz
--Geoff