The old web framework that we use is good for what it is, and we understand it, but its not state-of-the-art by any stretch, it does some weird things that aren't good practice in "modern" Perl, its still fairly heavily tied to Apache2/mod_perl and other parts of our internal libs, and so on. We don't use it for new code ourselves and we're actively migrating everything written in it onto Overture-based UI.
So given that even we don't want to use it and the off-the-shelf Perl web frameworks are generally better, it really doesn't help anyone to make it open source.
That's not to say there's nothing in our system not worth open-sourcing. For example, we've deliberately written our CalDAV/CardDAV libs in a nice clean modular way because we've planned to release them from the very beginning. Its just some things, like the web framework, that have no particular value.