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 threads::shared.
