why thread is not usable in perl5 of OpenBSD6.4?

I have to use thread on the perl5 of OpenBSD 6.4.
However, it was disabled on the distribution.
I tried to make the thread active to recompile the perl5 with -Dusethreads,
which led me to many test fails.

Why the thread function was disabled in this release?
Is it security reason?

Kenji