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return (a ({-href => $url. "?choice=checkout"},

sub get_checkout_link
{
...
    ($url = "https://www.addr.com/~summitwe/cgi-bin/pet_shop.cgi") =~ 
+s/^http:/https:/i;
    return (a ({-href => $url. "?choice=checkout"}, img ({-src => "../
+images/checkout.jpg",  -border => "0", escapeHTML ($label)})));
}

elsif ($choice eq "checkout")        # customer is ready to check out
{
    $page .= display_billing_form ($dbh, $cart_ref);
}

sub display_billing_form
{
...

    return ($page);
}