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

use XML::LibXML;

my $doc    = 'XML::LibXML'->load_xml( string => << '__XML__');
<?xml version="1.0" encoding="utf-8" ?>
<University>
	<students>
		<student id="1000"/>
		<student id="1001"/>
		<student id="1002"/>
	</students>
</University>
__XML__

my $query  = '//student[@id="1001"]';

for my $ele ($doc->findnodes($query)){
    my $new_ele = $doc->createElement('student');
    $new_ele->setAttribute('id', '1003');
    $ele->parentNode->insertAfter($new_ele, $ele) or die;
    last;            # Not needed if identifiers are unique. The whole looping is useless, then.
}

print $doc->toString;