$self->status('unknown') or $self->status('good') set an object's status, whereas $self->status('ungnown') generates an error. I would like this also to be available 

##</code><code>##

use strict;
use warnings;

my $a_or_b;
$a_or_b->{a} = 'a'; #should be okay
$a_or_b->{b} = 'b'; #should be okay
$a_or_b->{c} = 'c'; #should generate an error

##</code><code>##

$status->{good} = '1'; #should be okay
$status->{bad} = '1'; #should be okay
$a_or_b->{ungnown} = '1'; #should generate an error

##</code><code>##

use strict;
use warnings;

my($status);
tie $status, 'Status';

$status = 'ungnown'; # warning, and value does not get set.
$status = 'good'; # works

print "$status";

package Status;
use Tie::Scalar;
use Carp;
use base qw(Tie::StdScalar);

sub TIESCALAR {
	my $class = shift;
	my $status = shift;
	
	bless \$status, $class;
}

sub STORE
{
   my($me, $value) = @_;
   unless ($value eq 'good'){
      warn("invalid status: $value. Status not set");
	  return undef;
   }
   ${$me} = $value;
}

sub FETCH
{
   my($status) = @_;
   return ${$status};
}