I've never tried it, but you might want to: Font::TTFMetrics -- there's a portion of the man page that actually talks about how to work out the width of a string in a given font.

Update: Another thing to consider (might be overkill, but might be the easiest approach in some unexpected sense of "easy"): set up the post card as a PDF, using PDF::Create. I have tried that (though not for post cards), and it's quite usable.