Who knows???

I know

Let me repeat, you can't debug this piecemeal, its Re^11 and the problem is still unresolved,

If you want to actually figure it out you need to post a complete program

Or spend a week or a month reading docs, creating and debugging cgi programs, until you gain enough experience , to where you can post some short snippet, and figure out the problem

Look for example at these links and the links they link , and parents, and get a birds eye view of similar confusion
Re^3: CGI::Session keeps re-using same session ID
Re^5: Problems getting session management to work: is_expired seems to lie to me (cookies)
Re: perl CGI::Session not deleting the session

People are happy to try to debug stuff piecemeal ... its simply not a workable approach when both sides have knowledge holes