my $can_contain = sub {
    my ( $self, $object ) = @_;
    $self->container_type eq $object->container;
};

sub insert {
    my ( $self, $object ) = @_;
    unless ( $self->$can_contain( $object ) ) {
        die "I can't hold that object!";
    }
    push @{$self}, $object;
}