I'd say you should try contacting the maintainers of ImageMagick (not the Perl module) and find out if they'll divulge the secret of convert's color-reduction.
Jeff japhy Pinyan,
P.L., P.M., P.O.D, X.S.:
Perl,
regex,
and perlhacker How can we ever be the sold short or the cheated, we who for every service have long ago been overpaid? ~~ Meister Eckhart