Something like this I'd guess:

sub new {
    my $class   = shift;

    my $extends = $class->SUPER::new(@_);
    $extends->setFlyBehaviour( src::bo::CannotFly->new() );
    $extends->setQuackBehaviour( src::bo::CanSqeek->new() );
    
    my $self = {
        COLOR   => undef,
    };
    my @protected = qw/COLOR/;
    my @private   = qw//;

    my $closure = sub {
        my $field = shift;
        if ( exists $self->{$field} ) {
            grep( /$field$/, @private ) and caller(0) eq __PACKAGE__ 
                || confess "$field is private";
            grep( /$field/, @protected ) and caller(0)->isa(__PACKAGE_
+_)
                || confess "$field is protected";
            if (@_) { $self->{$field} = shift; }
            return $self->{$field};
        }
        else {
            return $extends->( $field, @_ );
        }
    };

    bless( $closure, $class );
    return $closure;
}

sub setColor {
    my $closure = shift;
    my $color   = shift;
    &{ $closure }("COLOR", $color);
}

sub getColor {
    my $closure = shift;
    return &{ $closure }("COLOR");
}
[download]

Ok, I have to repeat the incantation to mark the private and protected fields as such... for as long as I want to go that far. Here I don't use privates so I could drop the @private and the grep for private fields. In fact I could drop the grep as well and just test on the name before calling confess.

It would be neat to have a cpan class that would do this automagically though...