Looking at Imager, I searched for "PNG". A few matches down, I found PNG files - Imager::Files, "PNG" in Imager::Files. I see some mentions there of "compressed"; there might be something in that section to help you.... or there might not. If not, you might be able to find something in the source code. For future reference, official documentation and source code is probably a better first step then trying "every variation or combination I could think of"