O monks, how do I restrict a variable to a range of values?

I would like to be able to do something like

$self->status('unknown') or $self->status('good') set an object's stat
+us, whereas $self->status('ungnown') generates an error. I would like
+ this also to be available
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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
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I realize I could use fields qw(good bad unknown)and do something like

$status->{good} = '1'; #should be okay
$status->{bad} = '1'; #should be okay
$a_or_b->{ungnown} = '1'; #should generate an error
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However, this doesn't satisfy me because the status should not be allowed to be simultaneously good and bad.

I feel like this could be useful in a range of situations where I want to "program defensively against typos and the like. I have recently started using fields and hash::lock_keys to die when I make an $object->{misspelled} type mistake. This worked so well that my instinct is now to try this on individual variables. Is there a way to do this?

UPDATE: I guess I can do this with something along the lines of $self->set_status('ungnown') and validate against typos with grep like kwaping said. I just thought there might be some sweeter way to do this. But, the answer appears to be, there really isn't, other than *use accessors". So, I guess I will be using accessors...

UPDATE 2: Ha, I knew there was a way -- thanks jediwizard. Jedi presented a way with tied hashes, but what I really wanted was tied scalars, so here's what I'm using:

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};
}
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