sub _private_sub {
   die "_private_sub is private!" unless caller eq __PACKAGE__;
   # private stuff #
}

##</code><code>##

package Foo;

sub bar : public { ... }

package Bar;
use base 'Foo';

sub bar : private { ... }

package Baz;
use base 'Bar';


##</code><code>##

Baz->new->bar;