Looks like it well may be encoding, unless my fonts are all shagged.
I have printed $text to the command line and it comes out as the © symbol.
The text on the image has a square, indicating that the font doesn't recognise the symbol. I can't download Encode to this box, as it's running an old version of perl (5.6.1), and Encode needs a later version. I tried on my Fedora Core 2 box, which has an updated perl, but it won't install Image::Magick, so I'm still none the wiser.
For the purposes of now, I think I'll just create a jpeg of the © text I need and overlay it as a watermark.
Thanks for your input.