Thanks for that. I'd found the ReadImage example, but adapting it didn't get the results I was after - which may be because with Image Magick there always seems to be three different things that aren't clearly documented that you have to get right at the same time. I was confused by "ReadImage" which implies that there should be an image somewhere to read, but looks like it is actually a magical create image.
I have managed to overlay one image on another using burn in or dodge to get something like transparancy so I ought be able to do it by drawing to a second image then combine that image with the first. I'll post something in snippets when I get it sorted out.
Perl is Huffman encoded by design.