I don't see how XXE relates to doing XML validation that couldn't be addressed by limiting memory and CPU usage (which you would need to do either way), but you can nullify it by using XML::LibXML's load_ext_dtd and expand_entities options (as mentioned in the document you linked, under the heading "libxml2").

Rather than loading the entire document into memory, you'd want to use XML::LibXML's pull interface, XML::LibXML::Reader.

$ 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
[download]

$ 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
[download]