/descendant::Volume[@VolumeCategory="L"]

##</code><code>##

//Volume[@VolumeCategory="L"]

##</code><code>##

my @matching =
   grep {
      my $vc = $_->getAttribute('VolumeCategory');
      defined($vc) && $vc eq "L"
   }
      $doc->getElementsByTagName('Volume');

##</code><code>##

my @matching = $doc->findnodes('//Volume[@VolumeCategory="L"]');