So, I am inspired, and I have been messing around with fields. But I am seeing some what seems to me to be odd behavior. (I am running perl 5.8.0 installed on Mac OS X, so maybe I need to update, if so, let me know.)

package Brain;

use strict;
use warnings;
use fields qw/cells/;

sub new {
    my $pkg = shift;
    my $self = $pkg->fields::new();
    $self->{cells} = shift;
    $self;
}

package Dog;

use strict;
use warnings;
use fields qw/intelligence/;

sub new {
    my $pkg = shift;
    my $self = $pkg->fields::new();
    my Brain $brain = Brain->new(shift);
    $self->{intelligence} = $brain;
    $self;
}

sub getBrain {
    my Dog $self = shift;
    return $self->{intelligence};
}

package main;

use strict;
use warnings;

my Dog $dog = Dog->new(10);
my Dog $brain = $dog->getBrain();
my Brain $b = $brain;
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Sorry for this question, but my Programming Perl book does not go into too much detail (at least not that I have come across yet) and the Damian OO book is at the office (and I am not). And looking over the docs for fields it is not clear either.