If you did any homework, you would have gone to http://search.cpan.org/, typed "tree" in the box and found Tree. 
use Tree;

my @data = (
    [ 1,3 ],
    [ 1,4 ],
    [ 1,5 ],
    [ 3,8 ],
    [ 3,9 ],
    [ 4,11 ],
    [ 4,18 ],
    [ 8,14 ],
    [ 8,15 ],
    [ 5,17 ],
    [ 5,16 ],
    [ 17,21 ],
);

my %nodes;

foreach my $edge ( @data ) {
    my ($parent_id, $child_id) = @$edge;

    my $parent = $nodes{$parent_id} ||= Tree->new( $parent_id );
    my $child = $nodes{$child_id} ||= Tree->new( $child_id );

    $parent->add_child( $child );
}

[download]
Now, you have a tree. Do what you want with it.
My criteria for good software:
  1. Does it work?
  2. Can someone else come in, make a change, and be reasonably certain no bugs were introduced?

In reply to Re: representing a tree by dragonchild
in thread representing a tree by baxy77bax

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