any changes to the copied variables don't effect the values of those variables in other threads, i.e. everything is thread local.
Unless the cloned variables are closed over ....
You seem to be carving out an exception for something like this:
my $x = 20;
my $thread_id = shift;
say "In thread $thread_id: ", ++$x;
say "In thread 'main': $x";
In thread 1: 21
In thread 'main': 20
But even though the thread closes over $x, it cannot change the $x in main. So, it appears to me that the closed over variable is also thread local.