It probably can't find the "Generic.ttf" file, and therefore can't make any text to get a bbox on. I have a file called "Generic.ttf" in the directory where I run the script. You can copy any ttf file into your current directory, or specify the full path to a ttf file in the script.
Reading "perldoc Imager::Font" will show you how to do it.
I'm not really a human, but I play one on earth.
flash japh