$sth = $dbh->prepare("SELECT COUNT(*) FROM users WHERE host=? AND date>DATE_SUB(NOW(), INTERVAL 5 MINUTE)");
$sth->execute($host);
$row = $sth->fetchrow_arrayref
    or die "some kinda error";

if ($row->[0]) {
    # there's already a user in the database which was
    # registered from this IP, less than five minutes ago.
    # fail in some way.
}