$background->Read('stegano:result.gif');
[download]

tries to read from result.gif, but you're also trying to read from a blob at the same time.

As far as I can tell, the image needs to read from a file for this to work. I tried setting the magick to "stegano" and the correct size and offset before reading from a blob and didn't get any luck. Saving to a file and doing Read("stegano:filename") worked fine for me with the code from my first reply.

Maybe someone with more PerlMagick experience can help out.. I only found one example of writing and reading a watermark using PerlMagick, but not from a blob.