# @param string Mask, hex
# @param string Known number, hex
# @return array Suitabe numbers
sub by_mask_and_number {
    my ($mask, $known_number) = map{hex $_} @_;
    
    my $len = length sprintf "%x", $mask;
    my $max = hex("F"x$len);              # max number to testing
    my $on  = $mask & $known_number;      # bits must be set
    my $off = $mask & ~$known_number;     # bits must be unset

    grep{$on==($on & $_) && $off==($off & ~$_)} (1..$max);
}