The problem turned out, rather unexpectedly, to be that xscreensaver recommends the virtual package "perl". Since both perl-5.004 and perl-5.005 conflict with "perl" (providing "perl5" instead), dselect silently selected them for removal... which chained down to everything which had a direct or indirect dependency upon either. Ouch!!! Now that I've removed xscreensaver, everything's back to normal.