Comment by lizmat

In Perl 5 threading things are only "shared" semantically. In reality, there is an extra interpreter running where the "shared" variables live, and any value fetches and stores are handled with the `tie` interface (see http://www.perlmonks.org/?node_id=288022 for more information).

In Perl 6 on the other hand, everything is always shared and you can use high level constructs such as atomic increments, supplies, taps, `react` and `whenever` so you don't have to think about any (dead)locking issues as a developer.