Perl 5.6 introduced something called interpreter threads. Interpreter
threads are different from 5005threads (the thread model of Perl 5.005)
by creating a new Perl interpreter per thread, and not sharing any
data or state between threads by default.

Prior to Perl 5.8, this has only been available to people embedding
Perl, and for emulating fork() on Windows.

The threads API is loosely based on the old Thread.pm API. It is very
important to note that variables are not shared between threads, all
variables are by default thread local. To use shared variables one
must use threads::shared.

It is also important to note that you must enable threads by doing use
threads as early as possible in the script itself, and that it is not
possible to enable threading inside an eval "", do, require, or use.
In particular, if you are intending to share variables with
threads::shared, you must use threads before you use threads::shared.
(threads will emit a warning if you do it the other way around.)