my $root = do {
    our $S //= 0;
    srand $S if $S;
    my $r;
    my @a =(
        'a'..'z',
        1..9,
        'A'..'Z',
    );
    $r = int( rand $#a ), splice @a, $r, 2, [ @a[ $r, $r+1 ]] while @a > 1;
    @a = @{ $a[0] };
    \@a;
};