For attributes, use setValue, not setData: 
#!/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;

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Or, if you find it too verbose, here is the same in XML::XSH2:

open file.xml ;
set /root/parameters/key/@value[.="500"] 501 ;
save :b ;

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لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ

In reply to Re: Modifying XML with LibXML by choroba
in thread Modifying XML with LibXML by jasal

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