return html_page("Error: " . $access_token->to_string)
        if $access_token->{error};
[download]

You have not written a subroutine named html_page ... (you probably don't need one as you can use the Dancer2 page rendering or error page templates. But you cannot call a sub that does not exist in your code!)

Try printing out $access_token->{error} and see what the issue is.