I established a dictionary where each term can have children. The actual rooted structure is a tree. Or what Zaxo said. You have to watch out for circular references in my code.

my %dict; # A reverse index on %dict by id.
while (my $line = <$fh>) {
    my ( $id, $parent, $term ) = split ' ', $line, 3; # Fixed from -3 
+per sauoq

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

use Data::Dumper;
print Dumper( \ %dict );
[download]

Or with the circular reference problem nixed

use Util::Scalar 'weaken';
my %dict; # A reverse index on %dict by id.
while (my $line = <$fh>) {
    my ( $id, $parent, $term ) = split ' ', $line, 3; # Fixed per sauo
+q

    $dict{$id}{'id'}     = $id;
    $dict{$id}{'parent'} = $parent;
    push @{$dict{$parent}{'children'}}, $dict{$id};
    weaken $dict{$parent}{'children'}[-1];
}

use Data::Dumper;
print Dumper( \ %dict );
[download]