Maybe...

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

my $doc = XML::LibXML->new->parse_fh(*DATA);
for ( $doc->findnodes("//*") )
{
    $_->parentNode->removeChild($_)
        unless $_->textContent() =~ /\S/
        or $_->hasAttributes();
}
print $doc->serialize(1);

__END__
<?xml version="1.0" encoding="utf-8" ?>
<?meta name="GENERATOR" content="XML::Smart/1.6.9 Perl/5.010000 [cygwi
+n]" ?>
<mgdToneUpdate xmlns="http://www.blah.net/telcel/mgdTone" xmlns:blah="
+http://www.blah.com/content" xmlns:jcr="http://www.jcp.org/jcr/1.0">
  <blah:ppuid>blah602527000831</blah:ppuid>
  <thumbnailUpdate/>
  <prelistenUpdate>
    <prelistenMp3ResourceUpdate/>
    <prelistenWavResourceUpdate/>
    <prelistenSmafResourceUpdate/>
    <prelistenAmrwbResourceUpdate/>
    <prelistenRmfResourceUpdate/>
  </prelistenUpdate>
  <premiumUpdate>
    <mp3ResourceUpdate>
      <blah:resourceFilename/>
      <blah:resourceFilename>prelistenMTT00602527000831.mmf</blah:reso
+urceFilename>
      <blah:deviceId/>
      <blah:deviceId>blackberry9000_ver1</blah:deviceId>
      <blah:deviceId>nokia_e71_ver1_subua2</blah:deviceId>
      <blah:deviceId>mot_l6_ver1</blah:deviceId>
    </mp3ResourceUpdate>
    <wavResourceUpdate/>
    <smafResourceUpdate/>
    <amrwbResourceUpdate/>
    <rmfResourceUpdate/>
  </premiumUpdate>
</mgdToneUpdate>
[download]

Gives (some whitespace snipped)-

<?xml version="1.0" encoding="utf-8"?>
<?meta name="GENERATOR" content="XML::Smart/1.6.9 Perl/5.010000 [cygwi
+n]" ?>
<mgdToneUpdate xmlns="http://www.blah.net/telcel/mgdTone" xmlns:blah="
+http://www.blah.com/content" xmlns:jcr="http://www.jcp.org/jcr/1.0">
  <blah:ppuid>blah602527000831</blah:ppuid>
  <premiumUpdate>
    <mp3ResourceUpdate>
      <blah:resourceFilename>prelistenMTT00602527000831.mmf</blah:reso
+urceFilename>
      <blah:deviceId>blackberry9000_ver1</blah:deviceId>
      <blah:deviceId>nokia_e71_ver1_subua2</blah:deviceId>
      <blah:deviceId>mot_l6_ver1</blah:deviceId>
    </mp3ResourceUpdate>
  </premiumUpdate>
</mgdToneUpdate>
[download]