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

use XML::LibXML;

my $xml = 'XML::LibXML'->load_xml( string => << '======' );
<root>
<parameters>
<key name="Repetitions" value="500" />
<key name="Mode" value="COLD" />
</parameters>
</root>
======

my $root = $xml->documentElement;
my ($value) = $root->findnodes('parameters/key/@value[.="500"]');
$value->setValue(501);
print $root->serialize;