my $gd=$graph->plot(\@bar_data)->$format() or die $graph->error;
print '<img src=http://foobar.com/foobar.pl?image=$gd&format=$format>'
+;
[download]

$gd is a GD object, not the image itself. And you can't pass the object around as an URL parameter.

Likewise you can't pass the image itself around as an URL paramter, because it's massive binary data. Instead you have to create the image in the second script. Or you store it on disk in the first script, and just print the URL of the created image into the HTML.