use CGI::Session;

#initial trip to server
CGI::Session->name("SESSION_ID1");
my $session = new CGI::Session();
$session->param('user' => 'Fred');
print $session->header();

#subsequent trip
CGI::Session->name("SESSION_ID2");
my $session = new CGI::Session();
$session->param('user' => 'Barney');
print $session->header();

#later...
CGI::Session->name('SESSION_ID1');
my $session = new CGI::Session();
my $user_A = $session->param('user');

CGI::Session->name('SESSION_ID2');
my $session = new CGI::Session();
my $user_B = $session->param('user');

print "Content-type: text/html\n\n";
print $user_A."<br />";
print $user_B";