use DBI;
use CGI::Cookie;
my $dbh = DBI->connect("dbi:mysql:host=...;database=datenbank", "benutzer","pass");

my %cookies = fetch CGI::Cookie;
my $session_id = $cookies{sid}->value;
unless ($session_id) {
    ...
}

my $query = "SELECT guest FROM session WHERE session_id=?";
my $sth = $dbh->prepare($query);
$sth->execute($session_id);
my $row = $sth->fetchrow_hashref;
if ($row) {
    # $session_id vorhanden
    my $guest = $row->{guest};
...