#!/usr/bin/perl
use warnings;
use strict;

use XML::LibXML;

my $html = 'XML::LibXML'->load_html( string => << '__HTML__');
<form method="post" action="logout.lp" name="authform" id="logoutform">
        <input type="hidden" name="rn" value="-1383135969">
<form method="post" action="login.lp" name="authform" id="authform">
        <input type="hidden" name="rn" value="-1383135969">
__HTML__

my $value = $html->findvalue('//form[@action="login.lp"]/input[@name="rn"]/@value');
print $value, "\n";