Perl: threads vs JSON
JSON uses JSON::XS if installed which is not compatible with Perl threads (please don't take the author's words at face value - threads are discouraged and difficult to use effectively, but not deprecated and there are no plans to remove them). The community-preferred fork Cpanel::JSON::XS is thread safe and will be used by JSON::MaybeXS by default, which is a mostly drop-in replacement for JSON.