Actually, it would have been more correct to remove "shared" in this context. A non-shared variable is cloned when a thread is started. However, a shared variable is also cloned.

use threads;
use threads::shared;
my $foo : shared;
print "outside thread: foo = ".(\$foo)."\n";
threads->new( sub { print "inside thread: foo = ".(\$foo)."\n" } )->jo
+in;
__END__
outside thread: foo = SCALAR(0x107cc0)
inside thread: foo = SCALAR(0x232800)
[download]

Remember, a shared variable is nothing other than an ordinary tied variable, and cloned as any other variable when a thread is started. Whenever a thread accesses the value of a shared variable, its value is copied from the hidden shared variables thread into the thread local copy, and then presented to the outside world as a "normal" variable. Whenever a thread updates a shared variable, the value is updated in the thread local version, as well as in the hidden shared variables thread.

All of these copy actions should be COWed, ideally.

Hope this made sense.

Liz