I may be wrong, but I would expect any of the modules mentioned to return just an intensity in the case of a true grayscale image, I could be wrong, or it could be a case of the image technically being a color image that just so happens to only contain grays. Nonetheless, if all else fails, just use any one of the values, ignore the others, they should be the same.

Well, just did some testing, and at least in regards to Image::Magick it still returns RGBA values, I can't speak for the others haven't had much experience with them, so at least in the case of Image::Magick, I would suggest just dumping the spare values.