you need to understand what i said in Re^5: Cookie->fetch problem about how CGI::Session gives you back a new INTERNAL sessionid if it cant find the sessionid you give it or that session has expired. A SESSIONID IT COMPUTES ITSELF!!!
http://search.cpan.org/~markstos/CGI-Session-4.48/lib/CGI/Session.pm
If it fails, will create a new session id, which will be accessible through id() method.

id()

Returns effective ID for a session. Since effective ID and claimed ID can differ, valid session id should always be retrieved using this method.