I asked this a while back on comp.lang.perl.tk, and as it turns out there's not an easy answer. As I understand it now, there's no bitmap representation of the canvas anywhere: it's drawn on the fly.

Postscript is used because the same kinds of commands that can be used to draw on the canvas can be used to render postscript. (Postition HERE, put character THERE, draw line from HERE to THERE, put a bitmap over THERE, etc...)

Two techniques come to mind:

• (preferable) Capture it in Postscript, and use ghostscript to covert to a bitmap representation. I've had luck using this. Something like:

       gs -dNOPAUSE -sDEVICE=jpeg -sOutputFile=foo.jpg foo.ps
  [download]

  Should work okay.
• It might be possible to "scrape" it off the screen directly with a screen capture tool that has a command-line interface. The physical location of the canvas can be computed (you know where the window is, you know where the canvas is in the window, where's the canvas on the screen?) and that might be enough info. Haven't tried this and it sounds messy. But will work. :)