Fixes for perl 5.36 with multiplicity + memleak in $st->kv methods

I always keep messing up the aTHX_ and pTHX_ stuff because my system
perl isn't built with multiplicity, and I still haven't found a
satisfactory way of finding SV leaks. Valgrind can't track those :(

t/json_format.t

@@ -1,4 +1,5 @@
 use v5.36;
+use experimental 'builtin', 'for_list';
 use builtin 'true', 'false';
 use Test::More;
 use Tie::Array;