Most programmers who use Perl's object-oriented features construct their
objects by blessing a hash. But, in doing so, they undermine the
robustness of the OO approach. Hash-based objects are unencapsulated:
their entries are open for the world to access and modify.

Objects without effective encapsulation are vulnerable. Instead of
politely respecting their public interface, some clever client coder
inevitably will realize that it's marginally faster to interact directly
with the underlying implementation, pulling out attribute values
directly from the hash of an object.

WWW: http://search.cpan.org/dist/Class-Std/