=pod

package Canine;
package Dog;
my Canine $spot : Watchful ;

##</code><code>##

use attributes ();
attributes::->import(Canine => \$spot, "Watchful");

=cut

##</code><code>##

use warnings;
use strict;

package Dog;

sub MODIFY_SCALAR_ATTRIBUTES {
    my $class = shift;
    my $ref = shift;

    my %dog;
    @dog{@_} = (1) x @_;

    $$ref = \%dog;
    bless $$ref, $class;
    print "Woof!\n";
    ();
}

##</code><code>##

package main;

my Dog $spot :Watchful Muddy;

my $izzie = ref $spot;
print $izzie? "$izzie Spot is @{[keys %$spot]}": "No Spot", $/;

##</code><code>##

my Dog $rover;