Not bad, although assignment differs strongly from containment, so you wouldn't really want to say "Mary is the lamb" (my $mary = $lamb). How about this?

use strict;
use warnings;

my ($mary, $lamb) = (bless({}, 'Girl'), bless({}, 'Lamb'));

push(@{$mary->{possessions}}, $lamb);
$lamb->{attributes} = { size => 'little', fleece => 'white as snow' };
$lamb->{location} = \$mary->{location};

# And to test...
$mary->{location} = 'town';
use Data::Dumper;
print Dumper $mary, $lamb;
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"The dead do not recognize context" -- Kai, Lexx