my $q = CGI->new;

# this will return a session object.
# if the CGISESSID is avialable via cookie or param 
# that is what's used, otherwise it is a new session

my $session = new CGI::Session("driver:File", $q, {Directory=>'/tmp'});

# this prints a header with the CGISSESSID cookie:
print $session->header()

# now if you want to stick it in a form as well, get the SESSID
my $sessid = $session->id;

# and stick it in the form.....
print <<HTML;
<form>
<input type="hidden" name="CGISESSID" value="$sessid">
</form>
HTML