In this case it brings back all the values at the same time, such as "value1value2value3value4" etc... for an element.

I will admit that I am a beginner with respect to XML, but I think you took a leap of the imagination and you lost me.

Here is the program:

#!/usr/bin/perl

#  xml-libxml-ex1.pl      perl xml-libxml-ex1.pl file.xml  XML::LibXML
+ & namespaces
# From http://www.perlmonks.org/?node_id=1173200  Re: XML::LibXML & na
+mespaces by choroba on Oct 03, 2016 at 19:10 EDT

use warnings;
use strict;
use feature qw{ say };

use XML::LibXML;

my $dom = 'XML::LibXML'->load_xml(location => shift);
my $xpc = 'XML::LibXML::XPathContext'->new($dom);
my $rpt = 1;
$xpc->registerNs("ns$rpt", 'http://www.url.com/path/for/rpt');

my $nsdevices = "//ns$rpt:device";
my @devices = $xpc->findnodes($nsdevices);
say $_->getAttribute('timestamp') for @devices;

# This prints:
# 2016-01-01T00:00:00.000-00:00

__END__
[download]

Here is the input (which I had to make modifications to, in order to get the so-called "snippet" to work with E. Choroba's program.):

<?xml version='1.0' encoding='utf-8'?>
<Notify xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http:/
+/www.w3.org/2001/XMLSchema-instance" xmlns="http://docs.oasis-open.or
+g/wsn/b-2">
<NotificationMessage>
<Topic Dialect="http://docs.oasis-open.org/wsn/t-1/TopicExpression/Sim
+ple">TOPICNAME</Topic>
<ProducerReference>
<Address xmlns="http://www.w3.org/2005/08/addressing">address</Address
+>
<Metadata xmlns="http://www.w3.org/2005/08/addressing">
<ns2:MessageID xmlns:ns2="http://www.w3.org/2005/08/addressing">msgid<
+/ns2:MessageID>
</Metadata>
</ProducerReference>
<Message>
<ns1:rpt xmlns:ns1="http://www.url.com/path/for/rpt">
<ns1:reportObject>
<ns1:device timestamp="2016-01-01T00:00:00.000-00:00">
</ns1:device>
</ns1:reportObject>
</ns1:rpt>
</Message>
</NotificationMessage>
</Notify>
[download]

I named the input file file.xml

Here is the output:

2016-01-01T00:00:00.000-00:00
[download]

Sorry, but I see one value. Perhaps I am not playing with a full deck, i.e., you have a different input file than I have, and you are asking a question regarding output that you have obtained from your input file, but I can only guess what your input file looks like.

Anyhow, I was able to get E. Choroba's program to work on my machine which runs Strawberry Perl, and I have learned something from him, yet again.