Your regex matches because [^X] also matches H. Use [^XH] instead:

use strict;
use warnings;
use Test::More tests => 3;
my $re = qr{X[^XH]*?H[^HD]*?D};

my $str = "RXVXCHHHZHDT";

unlike("RXVXCHHHZHDT", $re);
like(  "RXVXCHBNDT",   $re);
unlike("RXVXCHHHZHDT", $re);
[download]