class variables in Moose

class-variables.pl

#!/usr/bin/env perl
#-*- mode: cperl -*-
use Modern::Perl;

use Person;

my $shelling = Person->new(name => "shelling");

my $count = $shelling->count;

say $count->value;              #=> 1

my $sherry = Person->new(name => "sherry");

say $count->value;              #=> 2

my $appollo = Person->new(name => "appollo");

say $count->value;              #=> 3

Counter.pm

use Modern::Perl;
use MooseX::Declare;

class Counter {
    has value => (
        is      => "rw",
        isa     => "Num",
        traits  => ['Counter'],
        default => 0,
        handles => {
            inc => "inc",
        },
    );
};

Person.pm

use Modern::Perl;
use MooseX::Declare;
use Counter;

class Person {

    has name => (
        is       => "ro",
        isa      => "Str",
        default  => "nobody",
        required => 1,
    );

    has count => (
        is      => "rw",
        isa     => "Counter",
        default => sub {
            state $count = Counter->new;
            $count;
        },
    );

    sub BUILD {
        my ($self) = shift;
        $self->count->inc;
    }

};