# if ($ENV{THREADS} && eval 'use threads; 1') {
# my $l = threads->create(\&_transform_half, $key_l, $seed, $rounds);
# my $r = _transform_half($key_r, $seed, $rounds);
# if ($ENV{THREADS} && eval 'use threads; 1') {
# my $l = threads->create(\&_transform_half, $key_l, $seed, $rounds);
# my $r = _transform_half($key_r, $seed, $rounds);