So I guess the "probably harmless" comment is a bit off the mark?

If you need to link to something that's identified by '-lqt-mt' then it is *not* harmless, and will definitely need to be addressed. But it *is* harmless if you *don't* need to link to something that's identified by '-lqt-mt' (eg if, instead, you have to link to something that's identified by '-lqt').

... the qt-3.8.8 path I'm using (via "--with-qt-dir=...")

Looking at the Makefile.PL, I'm wondering if you'd be better off specifying that location by amending $ENV{QTDIR} (in the Makefile.PL). You may also need to mess with the Makefile.PL's $MAKE. (See the INSTALL file, if you haven't already.)

Cheers,
Rob