Since Perl 5.8, thread programming has been available using a model called
_interpreter threads_ which provides a new Perl interpreter for each
thread, and, by default, results in no data or state information being
shared between threads.

(Prior to Perl 5.8, _5005threads_ was available through the 'Thread.pm'
API. This threading model has been deprecated, and was removed as of Perl
5.10.0.)

As just mentioned, all variables are, by default, thread local. To use
shared variables, you need to also load the threads::shared manpage:

use threads;
use threads::shared;

When loading the threads::shared manpage, you must 'use threads' before you
'use threads::shared'. ('threads' will emit a warning if you do it the
other way around.)

It is strongly recommended that you enable threads via 'use threads' as
early as possible in your script.

If needed, scripts can be written so as to run on both threaded and
non-threaded Perls: