I poked around a bit. I expect it is possible but I didn't see where/how. It looks like Imager harnesses pnglib which primarily does compression via zlib. I went through the pnglib docs quite a lot and it all seemed a mismatch for things I would call intuitive Perl; like setting the compression level or quality in the write call as you were trying. I also tried your code and quite a few other variations as guesses from the libpng doc and had no luck. I was also a little irritated and surprised by the arbitrary argument acceptance. :| If you figure it out or get an answer, please submit a doc patch. This would be really good to have documented.

You could maybe switch to jpg(?) in the short term. If you're not doing things that are either text oriented or clean deco-ish, jpg is a much better match for file size optimization than png.