foreach ($doc->findnodes('/library/cd/artist/text()'){
   my $artist = $_->data;
   if ($artist){
      print "$artist | ";
   }
   else {
      print "0 | ";
   }
}

##</code><code>##

foreach ($doc->findnodes('/library/cd/artist')){   #update: added ')'
   my $artist = $_->to_literal;
   if ($artist){
      print "$artist | ";
      #to change the text use my $textnode=$_->findnodes('./text()')
      # then use $textnode->setData('new text');
   }
   else {
      print "0 | ";
      # you can use $_->appendTextNode('text to add') here
   }
}

##</code><code>##

foreach ($doc->findnodes('/library/cd/artist[not(text())]'){