I know a little about ACS, but stopped using around the time OpenACS really got going. I can't say much about going from Perl to TCL, though, as I went the other way.
TCL syntax is a little odd and has one or two tricks up its sleeve, but really only one or two. It's a very easy language to learn, and much harder to get into trouble with than Perl. (So long as you have the good sense to avoid uplevel and upvar).
I think that the lack of CPAN will be greatly obviated by the fact that virtually all common functionality you might need to build a database-backed website is provided in one ACS package or another. Typically, the only place you will need to "extend" the system is for whatever it is that makes your site special and unique.