1 0 domestic_animal
2 1 dog
3 2 terrier
4 2 collie
5 3 fox_terrier

##</code><code>##

$in = "sample.txt";
open (IN, $in) or die "cant open the in\n";
#
my %dict; # A reverse index on %dict by id.
#
my %dict; # A reverse index on %dict by id.
while (not eof (IN)) {
	$line = <IN>;
    my ( $id, $parent, $term ) = split (' ', $line, 3) ;

    $dict{$id}{'id'}     = $id;
    $dict{$id}{'parent'} = $parent;
    push @{$dict{$parent}{'children'}}, $dict{$id};
}

use Data::Dumper;
print Dumper( \ %dict );

##</code><code>##

$VAR1 = {
          '4' => {
                   'parent' => '2',
                   'id' => '4'
                 },
          '1' => {
                   'parent' => '0',
                   'children' => [
                                   {
                                     'parent' => '1',
                                     'children' => [
                                                     {
                                                       'parent' => '2',
                                                       'children' => [
                                                                       {
                                                                         'parent' => '3',
                                                                         'id' => '5'
                                                                       }
                                                                     ],
                                                       'id' => '3'
                                                     },
                                                     $VAR1->{'4'}
                                                   ],
                                     'id' => '2'
                                   }
                                 ],
                   'id' => '1'
                 },
          '3' => $VAR1->{'1'}{'children'}[0]{'children'}[0],
          '0' => {
                   'children' => [
                                   $VAR1->{'1'}
                                 ]
                 },
          '2' => $VAR1->{'1'}{'children'}[0],
          '5' => $VAR1->{'1'}{'children'}[0]{'children'}[0]{'children'}[0]
        };