To delete a cookie, set the cookie again with ALL the same parameters used to set it initially, including the same cookie name, path, domain, "secure" flag (if any of these were used), and an expiration date in the past. That should do the trick.

But again, as someone else pointed out, this is also well-documented in the RFC. Or you can use the CGI.pm implementation to witness yourself what the cookie looks like when being deleted (i.e. re-set with exp. date in the past).