Short version (I might be wrong, since it's the first time I see this in use):

The import function calls _setup_symbols, which in turn calls _compile_all that calls _compile for each symbol in 'use' that has no corresponding function already defined. The _compile function does some magic and then calls _make_tag_func which builds "CGI::tagname" for unrecognized tagnames.
Whew ;-)

I don't have the time to read through all the details right now, but I like this mechanism. I'll add this to my todo list -- thanks for asking this question.

-mk