dbmopen %FOO, "my_database", 0644 or die;
...
sub foo :lvalue {
  my $self = shift;
  $FOO{$self->key};
}
...
my $obj = SubClass->new;
$obj->foo = 35; # calls tied(%FOO)->STORE($obj->key, 35);
my $fetch = $obj->foo; # calls tied(%FOO)->FETCH($obj->key);