Hi bjoern,
You would need to set the cookie to expire in the redirect response. In order to do this, you'd need to use HTTP::respond instead of HTTP::redirect so you can add a Set-Cookie header. Here is an example:
HTTP::respond 302 Location "http://newhost.example.com/redirect_location.html" Set-Cookie {cookie_name=null; Path=/; Expires=Thurs, 01-Jan-1970 00:00:00 GMT;}
The path needs to be set in this response with the same path that the original cookie was set with.
I haven't tested this exact command, but I've used similar before.
Aaron