Playing around with XML::LibXML::Attribute objects is usually unnecessary. It's easier to select the element and then use getAttribute and setAttribute, or if you have a vaguely recent copy of XML::LibXML, just treat the element as a hashref...

use strict;
use warnings;
use XML::LibXML 2;

my $xml = 'XML::LibXML'->load_xml(IO => \*DATA);

$_->{value}++
    for $xml->findnodes('//key[@name="Repetitions"]');

print $xml->toString;

__DATA__
<root>
<parameters>
<key name="Repetitions" value="500" />
<key name="Mode" value="COLD" />
...
</parameters>
</root>
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