Looks like the reworked C++ preferences start to work again.

t/threads.t

@@ -1,4 +1,4 @@
-use Test::More;
+use Test::More tests => 2;
 use strict;
 use warnings;
 
@@ -12,11 +12,6 @@ use List::Util qw(first);
 use Slic3r;
 use Slic3r::Test;
 
-if (!$Slic3r::have_threads) {
-    plan skip_all => "this perl is not compiled with threads";
-}
-plan tests => 2;
-
 {
     my $print = Slic3r::Test::init_print('20mm_cube');
     {