The easy way to test is to examine you stored session. Create your new session, check the expiry time in you database/file. Access your web page again and see if your expiry time has changed.

Now that you know HOW to check when you session is expired, you next need to look and see how you want your session to expire. If you are looking to check Idle time, well you need to reset the expiry value for the session EVERY time the user accesses the session

my $session = CGI::Session->load();

if ($session->is_expired) {
   # Do something like refresh the page
}
else {
   $session->expiry("+5m");
   # Wait another 5 minutes
}
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There is nothing magical about how CGI::Session handles expiration. You set an initial expiration and that is it. By default, it will not reset the expiration value for you. There is no hidden value that deals with idle time.