sub cross (*$f, *@a) {
    return unless $f;
    @a or return @{$f}.map:{[$_]} ;            # Handle one argument.
    my @r = cross(@a);                        # Recurse.
    @$f.map:{my $l = $_ ; @r.map:{[$l, @$_]} }; # Distribute.
}

cross().perl.say;
cross([],[1]).perl.say;
cross([1,2]).perl.say;
cross([1,2],[3]).perl.say;
cross([1],[2],[3]).perl.say;
cross([1],[2],[3,4]).perl.say;