Greatest value of what? NICK?, ROWID?, RANK?

If you just want to get the last node in the list:

#!/usr/local/bin/perl

use XML::XPath;
my $file = 'test.xml';
my $xp = XML::XPath->new(filename=>$file);
my $nodeset = $xp->find('/MEMBERLIST/LIST[position()=last()]');

foreach my $node ($nodeset->get_nodelist) {
  print "FOUND\n\n",
  XML::XPath::XMLParser::as_string($node),
  "\n\n";
}
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If you want the greatest value of a particular field then you're going to have to sort by that field first. I think that's the point where you run into limitations with XML::XPath (and I would recommend a combination of XML::LibXML and XML::LibXSLT.