You can test what is returned from the text node. There is an xpath expression to do this also but I usually just test it like this. (untested code)

foreach ($doc->findnodes('/library/cd/artist/text()'){
   my $artist = $_->data;
   if ($artist){
      print "$artist | ";
   }
   else {
      print "0 | ";
   }
}
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If the artist was formerly called Prince I'm not sure whether Perl will evaluate that symbol to true or false, (or male or female)... it's a little confusing.

Update: The above won't work since findnodes() will only return the nodes that have text. I often have the situation where I need to add text to empty text nodes. Here is something closer.

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
   }
}
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Another option is to loop through seperately the nodes with no text using the not() funtion of xpath.