using an object as a variable type

[Random_Walk]

Honoured bretheren in Perl

I am trying to create a constrained variable using syntax like this ...

use threads;
use threads::shared;
my $var : shared;

To keep things simple I have created a package called Me providing a 'Me' object. Now I want to be able to do something like this.

use Me;
my $user : Me;
$user = "Random"; # this is OK
$user = "Deterministic"; # croaks

Here is the Me.pm

#!/usr/bin/perl
use strict;
use warnings;

package Me;
use Carp;
use overload '""' => \&str;

sub new {
    my ($class, $value) = @_;
    validate ($value) if defined $value;
    bless \$value, $class;
}

sub validate {
    my $value = shift;
    my @names = qw (Random_Walk Random Walk);
    croak "$value is not a good name\n" unless grep {$value eq $_} @na
+mes;
}

sub str {"Not telling you"};
1;

It works fine like this

> perl -Mlib=. -MMe -le'my $m = new Me (Random);print $m'
Not telling you
> perl -Mlib=. -MMe -le'my $m = new Me (Determined);print $m'
Determined is not a good name
 at -e line 1
# But I can not suss out how to get his to work
> perl -Mlib=. -MMe -le'my $m : Me; $m = "Random"; print $m'
Invalid SCALAR attribute: Me at -e line 1

I have had a look into attributes and found this Invalid SCALAR attribute? but I remain completely unenlighted how to use this to get the threads::shared syntax working or even if it is possible without delving into XS

Cheers,
R.