if you're on unix just copy my patch into a file, find where the AuthorizeNet.pm is located and do patch Authorize.pm copied_patch_file should do the trick (backup first though!)

If you're on Windows, I don't how to patch there, I'm sure there's a utiltity for it out there somewhere, if you don't figure it out just /msg me with your email and I'll send you a copy that's been patched.