Some options (in no particular order):

• Use a proper OO language ;-)
• Wait for Perl 6
• Use inside out objects (for me, the neatest discovery from perlmonks so far - ++Abigail-II).
• Hide your objects implementation behind a proxy object (Class::Delegation is handy for this sort of thing).
• Document that you use a hash. Document the slot(s) that you use. One idiom that I've seen a few times is to do it as $object->{CLASS_NAME}->{SLOT_NAME}

Any more for any more?