Thanks. I don't know why it didn't occur to me that perl5db.pl would be listed in %INC. I was, in fact, still including the wrong one, apparently because of the quotes aroung the path in the value for PERL5LIB. I fixed that, and it is now including the right one, but it's still griping about not recognizing the option.
I guess it's time to dig into the source for ActiveState's version of the debugger and see if anything leaps out at me.