Finally, after considerable largely redundant and repetitive effort, mod_perl is installed and running.
The biggest problem was in my initial installation of MovableType:I installed *everything* in /www/movabletype and ScriptAliased that directory so the cgi processes would be able to run. That was the beginning of a slow and inexorable ass kicking . . . . all self-inflicted.
What I *should* have done was installed the executables somewhere else (like /www/mt and the html, images and css stuff in /www/movabletype. Of course the documentation doesn’t spell out the the lib, extlib, and tmpl directories also have to live where the executables live.