> You should probably make some of the slots weak.

of course, but ...

I hope this makes it clearer now:

#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

{   package Element;
    use Moo;

    has name => (is => 'ro');
    has active_container => (is => 'rw');
}


{   package Container;
    use Moo;

    has elements => (is => 'ro', default => sub { [] });

    sub add_elem {
        my ($self, $element) = @_;
        push @{ $self->elements }, $element
            unless grep $_ == $element, @{ $self->elements };
    }

    sub get_elem {
        my ($self, $name) = @_;
        my $elem = (grep $_->name eq $name, @{ $self->elements })[0];
        $elem->active_container($self);
        return $elem
    }
}


my $elem0 = 'Element'->new(name => 'one');
my ($cont1, $cont2) = map 'Container'->new, 1, 2;
$cont1->add_elem($elem0);
$cont2->add_elem($elem0);

my $elem1 = $cont1->get_elem('one');
say "OK" if $elem1->active_container == $cont1;
my $elem2 = $cont2->get_elem('one');
say "OK" if $elem2->active_container == $cont2;

print "Not OK" if $elem1->active_container != $cont1;
[download]

C:/Strawberry/perl/bin\perl.exe -w d:/tmp/pm/container_elem.pl 
OK
OK
Not OK
Compilation finished at Fri Oct  8 17:48:07
[download]