# generate either HTML or graph depending on CGI parms

    my $graph_parms = (new CGI)->param('graph');
    if ($graph_parms) { gen_graph($graph_parms) }
    else              { gen_html()              }

    sub gen_html {
        # ... generate other HTML ...
        # add IMG element that calls back to us to get image
        print qq(<img src="index.cgi?graph=..." ...>);
    }

    sub gen_graph {
        my $parms = shift;
        my $img_data = generate_graph( $parms );
        print "Content-Type: image/png\n",
              "Cache-Control: max-age=300\n\n",
              $img_data;
    }