Chalk up another vote for HTML::Template. Works equally well with CGI and mod_perl, since it's only involved in the content delivery phase. It doesn't give you all the bells and whistles of the other frameworks; then again, it's a single perl module that you can install yourself on any server that lets you run perl CGI scripts. There are may ways to handle your own access/authorization control and logging in a CGI environment, and with a little thought and planning you should be able to come up with something that you can convert easily to mod_perl when the time comes.

I'm using HTML::Template right now, and I recently found out for real just how nice it is to separate the design from the code when a designer gave the application I'm working on a facelift. I didn't have to change a single line of my code. She didn't have to learn anything about perl. I didn't have to stop working on my stuff while she made changes. In fact, I didn't even know she was doing it until she plopped the new templates in place and the pages appeared with the new design!