sub _build {
    my ($self, $file) = @_;
    croak "Unable to retrieve data" unless open(INPUT,$file);

    # You just stomped on the global $_. You have to either assign it
    # to a pad variable (like I did here) or local $_ first. Just a
    # plain while(<FH>) is dangerous.
    while (my $line = <INPUT>) {
        # You didn't pick named variables. Variables should have meaningful
        # names so you don't have to guess at what $field[2] is later in
        # the code.
        my ($id, $parent_id, $title) = split ' ', $line;
        unless ( $id =~ /^\d+$/ and
                 $parent_id =~ /^\d+$/ ) {
            carp "Unable to parse\n$line";
            next;
        }

        # You were assigning the id to descendants. That should be stored
        # separately. Also, you used the hash keys of the object for the
        # node titles but also expect the 'decendents' key to remain unused.
        # You should keep that separate somewhere.
        $self->{'title2id_dict'}{$title} = $id;

        # Shall we assume the IDs all start low and go up? You'll consume
        # much memory if an ID comes in like 1000000. I compensate for that
        # and just use a hash.
        push @{$self->{'descendants'}{$parent_id}}, $id;
    }
}