POSIX::_exit() calls your C-runtime to terminate the process. That bypasses all Perl contructs including END{} blocks and DESTROY() subroutines. Effectively, it completely bypasses Per's usual clean-up routines.

In doing so, it bypasses the perl-core code that detects that there are still threads running and produces the warning. But, it can also give problems if you are using modules that use DESTROY subs or END blocks to persist data, flush buffers and the like.

You can often cause object DESTROY methods to be called by undefing the handles, but I don't know of any way of forcing END blocks to be run without going through the full perl clean-up process.

In other words, use _exit() with caution.