This site has a decent explanation of cookie mayhem.

A snippet from that site states:

The URL must use an absolute path; otherwise, the web server may attempt to avoid another request and response cycle by simply returning the content for the new URL as the content of the initial response via an internal redirect.

perldoc CGI gives:

script_name() Return the script name as a partial URL,
              for self-refering scripts.
[download]

Hope this helps,

Edited by davido: Disabled link to website violating O'Reilly's copyright.