I can't really comment as I gave up trying to get Image::Magik going on my machine after 3 attempts. I rarely have the need to automate image stuff, and have (what I consider) to be superior tools available for doing this sort of thing manually.

(If you've never seen the performance and tightness of the Xara graphics tools, particularly Xara X, take a look, they have free demos available for download. Note:Just a very satisfied user. No commercial or personal relationships with the company.)

Anyway, my speculation would be that at some point, the image matrices in Image::Magic are stored and manipulated using perl array's, possibly they are expanded on demand rather than preallocated and it is the repeated, incremental increase in the size of these arrays that is responsible for the memory (TIFFs can be very memory hungry) consumption, and consequently the cpu consumption.

Manipulating large, but relatively static chunks of binary image data is one of the few applications where perl's arrays aren't really of benefit.