package Baz;

sub new {
   my ($class) = @_;
   return bless({}, $class);
}

sub example {
   my ($self) = @_;

   if (ref($self) ne __PACKAGE__) {
      require Carp;
      Carp::croak(
         "Method &" . __PACKAGE__ . "::example must not be inherited"
      );
   }

   print(ref($self) . " ok\n");
}


package Baz::Foo;

@ISA = 'Baz';


package main;

Baz     ->new()->example();  # ok.
Baz::Foo->new()->example();  # dies.