Well, I'm kinda unclear as to what the exact problem is. Did the original have the content in the center? It would seem likely, that if this is not the case, that this is a display issue with Acrobat (or the full script is doing more manipulation than the snippet shown above is). Otherwise perhaps "Crop" is your friend.
Anyhow for all of you Image::Magick needs this page is, if you haven't already found it, a life saver. Particularly the section on "Manipulating an Image".