$ cat a.xml
<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE foo [
   <!ELEMENT foo ANY >
   <!ENTITY xxe SYSTEM "file:///etc/passwd" >]><foo>&xxe;</foo>

$ perl -MXML::LibXML::Reader -e'
   my $reader = XML::LibXML::Reader->new(
      location        => $ARGV[0],
      load_ext_dtd    => 0,
      expand_entities => 0,
   );
   while ($reader->read) {
      printf("%d %d %s\n",
         $reader->depth,
         $reader->nodeType,
         $reader->name,
      );
   }
' a.xml
0 10 foo
0 1 foo
1 5 xxe
0 15 foo

##</code><code>##

$ cat bad.xml
<foo><bar></foo>

$ perl -MXML::LibXML::Reader -e'
   exit 1 if !eval {
      my $reader = XML::LibXML::Reader->new(
         location        => $ARGV[0],
         load_ext_dtd    => 0,
         expand_entities => 0,
      );
      1 while $reader->read;
      1
  };
' bad.xml

$ if [ $? -eq 0 ]; then echo "well-formed" ; else echo "error" ; fi
error