Because I only had one key, I took shortcuts over how I'd implement a more involved object. It's just the implementation, so it can be changed as needed. With a dispatch table, it's pretty obvious that there's a straightforward parallel for any changes you make in your system. 
sub STORE {
  my ($self, $key, $val) = @_;
  # An inline dispatch table
  ({
    foo => sub {
      $self->[1] = localtime(); # track last change
      $self->[0] = $val;
      $self->_recalculate_bar();
    },
    # ...any other keys...
   }->{$key}
   or sub {croak (ref $self) . " has no $key member"}
  )->();
}

sub FETCH {
  my ($self, $key, $val) = @_;
  ({
    foo => sub {
      $self->[2] = localtime(); # track last access
      return $self->[0];
    },
    # ...any other keys...
   }->{$key}
   or sub {croak (ref $self) . " has no $key member"}
  )->();

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In reply to Re^10: tie for Perlish, encapsulated objects by Roy Johnson
in thread tie for Perlish, encapsulated objects by Roy Johnson

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