I haven't tried this yet, but rather than using CPAN try using macports. Not sure if you are familiar with it... but there is a GD2 port that I think would meet the requirement of mobiperl.
It looks like you would need to install the following macports to get mobiperl working.
gd2
p5-palm
p5-timedate
p5-getopt-mixed
p5-image-size
p5-xml-parser-lite-tree
p5-encode
BOb
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