$ perl -Ilib -V:config_args -V:useithreads
config_args='-de -Dusedevel -Dusethreads';
useithreads='undef';

$ perl -Ilib -E'use threads; say "threaded"'
This Perl not built to support threads
Compilation failed in require at -e line 1.
BEGIN failed--compilation aborted at -e line 1.