sub STORE {
  my ($self, $key, $val) = @_;
  # assuming this class has more than just 'foo'...
  unless (grep { $_ eq $key } @CLASS_KEYS ) {
    croak (ref $self) . "has no $key member";
  }

  if ($key eq 'foo') {
    $self->[1] = localtime(); # track last change
    $self->[0] = $val;
    $self->_recalculate_bar();
  } else {
    # set normal keys somehow
  }

}

sub FETCH {
  my ($self, $key, $val) = @_;
  # assuming this class has more than just 'foo'...
  unless (grep { $_ eq $key } @CLASS_KEYS ) {
    croak (ref $self) . "has no $key member";
  }

  if ($key eq 'foo') {
    $self->[2] = localtime(); # track last access
    return $self->[0];
  } else {
    # get normal keys somehow
  }
}