Just to be clear, I said the global my_perl isn't used by the THX macros with or without MUTLIPLICITY.

As for MULTIPLICITY being the default? No. It's is off by default.

MULTIPLICITY is what allows a process to have multiple interpreters. A build with thread support requires MULTIPLICITY since each thread has its own interpreter. So MULTIPLICITY is automatically defined when a building Perl with thread support.

A build without thread support usually won't use MULTIPLICITY. If the OP's program only needs one interpreter, they should use a build of Perl that was built without MULTIPLICITY (for a performance boost).