Note the input data you've shown isn't valid XML, and you haven't shown the expected output, so I have to guess at what you want. See How do I post a question effectively? and Short, Self-Contained, Correct Example.

As documented, ->findnodes in scalar context returns an XML::LibXML::NodeList object, which you can't pass to ->createElement. Here's one approach:

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

my @files = ("file1.xml", "file2.xml", "file3.xml");
my $outfile = "result.xml";

my $newdom = XML::LibXML::Document->new('1.0', 'UTF-8');
my $root = $newdom->createElement('testsuites');
$newdom->setDocumentElement($root);

for my $file (@files) {
    my $dom = XML::LibXML->load_xml(location => $file);
    for my $node ( $dom->findnodes('//testsuite') ) {
        $node->{name} =~ s/test_suite_name_/tsname_/;
        $root->appendChild($node);
    }
}

$newdom->toFile($outfile, 1);
[download]

With input files that look like this:

<?xml version="1.0" encoding="UTF-8"?>
<testsuites>
  <testsuite name="test_suite_name_one">
    <testcase name="test1" />
    <testcase name="test2" />
  </testsuite>
</testsuites>
[download]

It produces output like this:

<?xml version="1.0" encoding="UTF-8"?>
<testsuites>
  <testsuite name="tsname_one">
    <testcase name="test1"/>
    <testcase name="test2"/>
  </testsuite>
  <testsuite name="tsname_two">
    <testcase name="test3"/>
    <testcase name="test4"/>
  </testsuite>
  <testsuite name="tsname_three">
    <testcase name="test5"/>
    <testcase name="test6"/>
  </testsuite>
</testsuites>
[download]