NetWallah already told you what was the reason of the error. I would like to add a piece of advice:
Do not go over the elements in a loop (unless you want to add a student to almost each of them). Use XPath to search for the desired target:
#!/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;
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