The problems start with this line   print header,start_html(-title=>'Bright', -bgcolor=>'#BDBDB0'); which sends a MIME type of "text/html" back to the browser, then starts an HTML page. At this point, it's a bit late to start printing image bits and expect the browser to display an image.

Read the description of header() in the CGI.pm POD, and note how to override the MIME type (aka Content-type). You want to send "image/png".

I should have given up and gone to bed hours ago.

Is it possible that there's something in $start or $finish that is fouling up the shell meta-character expansion that happens when you use the single-argument form of exec(), preventing the command from generating an image?

Try using the list form of exec(), which doesn't trigger shell meta-character expansion.