# returns a list of vectors of numbers in (0,1)
sub gray2;
sub gray2
{
    my($i) = @_;
    $i <= 1 and return([0],[1]);
    my @a = gray2($i-1);
    ( ( map { [ 0, @$_ ] } @a ), map { [ 1, @$_ ] } reverse @a )
}

my @a = gray2(3);
print "@$_\n" for @a;