Ok, I think I know what you mean now. You want something like this (doesn't work because CGI::Session doesn't support it):

$session = CGI::Session->new(undef, $sid, {Directory=>'C:/apache/sessi
+ons',NoCreateIfNotExists=>1});
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I'm not entirely clear on the architecture of CGI::Session, but that may need to go into the DSN instead. Anyway, once you tell it not to autocreate new sessions, the code would have to change to honour it.

Another alternative, which I just stumbled upon. Which you may not like for similar reasons as before. I suspect this is how the author of CGI::Session envisions doing what you ask for:

my $sid = $foo->cookie('main') || undef;
my $session; # = undef
if (defined $sid) {
  $session = CGI::Session->new(undef, $sid, {Directory => 'c:/apache/s
+essions'});
  $session->delete() if $session->is_new();
}
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When you get the session, you can check if it's a new session or not. If it is, you can delete it. From what I can tell reading the code, this won't hit your hard disk at all. And the fact that the is_new sub is there at all says to me that this is the author's idea of solving this very problem.