Paul,
It's a strange behavior and i think it has nothing to do with your installed tcl version.
As far as i know mime package for acs-mail-lite is provided by acs-tcl. Look at file packages/acs-tcl/tcl/mime-procs.tcl. There is an approved TIP (https://openacs.org/forums/message-view?message_id=298282 ) to get rid of all old code in OpenACS that is has better equivalents in TCLlib (e.g: mime-procs) but i think it will be implemented only in future 5.3.
Please try restart your service and look for errors while loading acs-tcl procs. Anyway try install tcllib software (http://tcllib.sourceforge.net)
