#!/usr/bin/perl -w
use strict;
use warnings;
use Data::Dumper;

my (@child_list, %Nodes, $id, $data, $parent);
while (<DATA>) {
    @{$Nodes{$id}}{'ID','Data','Parent'} = (($id,$data,$parent)=split /\s+/,$_);
    $Nodes{$parent}{Children}{$id} = $Nodes{$id};
    push @child_list, $id;
}
delete $Nodes{$_} for @child_list;

print Dumper(\%Nodes);                                                                    
__DATA__
1    data    0
2    data    1
3    data    1
4    data    3
5    data    4