#!/usr/bin/perl
    use strict;
    use warnings;
    use Data::Dumper;
    
    my @arr = (
            'ng1',     ['ng1_1','ng1_2', 'ng1_3', 'ng1_4'],
            'ng2',     ['ng2_1','ng2_2', 'ng2_3', 'ng2_4'],
            'ng3',     ['ng3_1','ng3_2', 'ng3_3', 'ng3_4'],
            'ng1_1',   ['ng1_1_1','ng1_1_2', 'ng1_1_3', 'ng1_1_4'],
            'ng1_1_1', ['ng1_1_1_u1', 'ng1_1_1_u2', 'ng1_1_1_u3'],
            'ng2_1',   ['ng2_1_u1', 'ng2_1_u2', 'ng2_1_u3']
    );
    
    my @tree;
    
    #print "\nBEFORE CALLING FIRST FOR LOOP\n";
    #print Dumper @arr;
    
    $tree[0] = $arr[0];
    $tree[1] = $arr[1];
    for (my $i=2; $i < @arr; $i+=2){
      &buildTree(\@tree, $arr[$i], $arr[$i+1]);
    }

    #print "\nAFTER CALLING FIRST FOR LOOP\n";
    #print Dumper @arr;
    
    #$tree[2] = $arr[2];
    #$tree[3] = $arr[3];
    #for (my $i=4; $i < @arr; $i+=2){
    #  &buildTree(\@tree, $arr[$i], $arr[$i+1]);
    #}
    
    sub buildTree{
      my ($tree, $parNg, $subNg) = @_;
      for my $treeElement (@{$tree}){
        if (ref $treeElement eq "ARRAY"){
          &buildTree($treeElement, $parNg, $subNg);
        }
        else{
          if ($treeElement eq $parNg){
            my ($index) = grep { $tree->[$_] eq $treeElement } 0..scalar(@$tree)-1;
            splice @{$tree}, $index + 1, 0, $subNg;
          }
        }
      }
    }