mp3Resource is {http://www.Borkbot.net/telcel/Bork}mp3Resource, as specified in the root node.

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

use XML::LibXML               qw( );
use XML::LibXML::XPathContext qw( );

my $file = $ARGV[0];

my $parser = XML::LibXML->new();
my $dom    = $parser->load_xml( location => $file );
my $root   = $dom->getDocumentElement();
my $xpc    = XML::LibXML::XPathContext->new($root);

$xpc->registerNs(t => 'http://www.Borkbot.net/telcel/Bork');
$xpc->registerNs(c => 'http://www.Borkbot.com/content');

my @nodes = $xpc->findnodes('//t:mp3Resource/c:resourceFilename');
foreach my $node (@nodes) {
    print($node->to_literal, "\n");
}
[download]

Update: Tested. Had wrong namespace. Misused registerNs. Fixed.