rot13 in Perl 6

rot13. not as simple as I'd hoped

sub rot13($s) {
    join "", map { chr(do given ord($_) {
        when ord('A')..ord('Z') { ($_ - ord('A') + 13) % 26 + ord('A') }
        when ord('a')..ord('z') { ($_ - ord('a') + 13) % 26 + ord('a') }
        default { $_ } })
    }, $s.comb
}

say(rot13($_)) for
    q[V'ir zrnfherq vg sebz fvqr gb fvqr:],
    q['Gvf guerr srrg ybat, naq gjb srrg jvqr.]