Lvalue Subroutines

When accessors define bits of data that change dynamically during the object's lifetime, it sometimes makes sense to define attribute-like access semantics:

    sub foo : lvalue {
      my ($self) = @_;
      $self->{foo};
    }

Caller says:

    print $object->foo; # Prints "my foo".
    $object->foo = "new foo"; # Change foo back.