Thanks Zentara, the gtk-perl list looks like where I should go. I've tried various versions of the sources, both from Mandrake and direct from CPAN, so it isn't Mandrake's fault.
Comment on Re^2: Perl Glib build fails on G_PARAM_PRIVATE
Fixed - I had messed up my glibc system libraries - I think libs and headers were out of sync. Reinstalled and now all is well. This also seems to account for a few other weird errors I was getting like compile segfaults.