Don't forget there is no referrer if you're loading a page directly. You have to have clicked through something. Try putting a dummy link to stay in your CGI and see.

use CGI qw(:standard);
print header(), start_html(); # <-- update to run/test w/o changes
print a({-href=>url(1)},
        "self-check: ",
        referer() || 'no referrer');
[download]