#!/usr/bin/perl
use strict;
use HTML::Template;
use Data::Dumper;

my $template = "
<ul>
  <TMPL_LOOP tree>
  <li><TMPL_VAR value> (depth=<TMPL_VAR depth>)
      <TMPL_LOOP up><ul></TMPL_LOOP>
      <TMPL_LOOP down></ul></TMPL_LOOP>
  </li>
  </TMPL_LOOP>
</ul>
";

my $tree = [
        { 'value' => 'a',    'depth' => 1 },
        { 'value' => 'b',    'depth' => 1 },
        { 'value' => 'b1',   'depth' => 2 },
        { 'value' => 'b2',   'depth' => 2 },
        { 'value' => 'c',    'depth' => 1 },
        { 'value' => 'c1',   'depth' => 2 },
        { 'value' => 'c1.1', 'depth' => 3 },
        { 'value' => 'd',    'depth' => 1 }
];

for my $i (0 .. $#$tree) {
       if (defined $tree->[$i+1]) {
          my $diff = $tree->[$i+1]->{depth} - $tree->[$i]->{depth};
          if ($diff > 0) {
            $tree->[$i]->{up} = [({}) x $diff];
          } elsif ( $diff < 0 ) {
            $tree->[$i]->{down} = [({}) x -$diff];
          }
       }
}

my $html_template = HTML::Template->new(
        scalarref => \$template,
        loop_context_vars => 1);

$html_template->param('tree' => $tree);
print $html_template->output;