I'm sorry, I forgot to show how to write out the image, silly me!
file => $tmp->filename,
type => 'tiff',
class => 'fax',
And then in order to get the Group4 compression I did resort to ImageMagick's convert util.
system("convert -page Letter -compress Group4 $filename $filename")
The last part I'm still not completely happy with, but since this is a batch process I didn't have to worry so much about having the extra system call.
-- More people are killed every year by pigs than by sharks, which shows you how good we are at evaluating risk. -- Bruce Schneier