In that example you're not actually sharing the data: you're creating a new hash every time data() is called.

How unfortunate I am ! I can't have all my bad code reviewed by your sharp eyes, precious friend. Thanks a lot for this one :)

update: in the example you posted here, there's no need to do
my $proto = ref $self;
print $proto->data;

Yes, this one is just a hurried copy-and-paste :)