#!/usr/bin/perl

# http://perlmonks.org/?node_id=1142838

use strict;
use warnings;
$| = 1;

$_ = join '', <DATA>;

my ($fourth) = /(?:.*?<li>(.*?)<\/li>){4}/s;

print "$fourth\n";

__DATA__
<ul>
<li><strong>site_user</strong></li>
<ul>
<li>user1</li>
</ul>
<li><strong>compare_hidden</strong></li>
<ul>
<li>average_speed_answer  25 60 30 60 ^M<br />
 calls_waiting  300 500 300 500 ^M<br />
 many more rows here
 post_ivr_calls_handled Wisconsin 50 100 50 100 ^M<br />
 post_ivr_calls_handled Wyoming 50 100 50 100 ^M<br />
</li>
<li><strong>calls_waiting_good_high</strong></li>
<ul>
<li>300</li>
</ul>
<li><strong>calls_waiting_warning_low</strong></li>
<ul>
[download]

Of course, this will have problems if you have nested li's