https://rt.cpan.org/Public/Bug/Display.html?id=97458
Avoid warning with newer Perl.
Reference is already weak at lib/Net/IPTrie/Node.pm line 60.

Index: lib/Net/IPTrie/Node.pm
--- lib/Net/IPTrie/Node.pm.orig
+++ lib/Net/IPTrie/Node.pm
@@ -4,7 +4,7 @@ use warnings;
 use strict;
 use Carp;
 use Class::Struct;
-use Scalar::Util qw(weaken);
+use Scalar::Util qw(weaken isweak);
 use vars qw($VERSION);
 $VERSION = '0.7';
 
@@ -56,7 +56,9 @@ Net::IPTrie::Node
 
 sub new {
     my $ret = shift->SUPER::new(@_);
-    if ( defined($ret->{'Net::IPTrie::_Node::up'}) ) {
+    if ( defined($ret->{'Net::IPTrie::_Node::up'}) &&
+	!isweak($ret->{'Net::IPTrie::_Node::up'}) )
+    {
 	weaken $ret->{'Net::IPTrie::_Node::up'};
     }
     return $ret;
@@ -66,7 +68,9 @@ sub up {
     my $self = shift;
     if (@_) {
 	$self->{'Net::IPTrie::_Node::up'} = shift;
-	if ( defined($self->{'Net::IPTrie::_Node::up'}) ) {
+	if ( defined($self->{'Net::IPTrie::_Node::up'}) &&
+	    !isweak($self->{'Net::IPTrie::_Node::up'}) )
+	{
 	    weaken $self->{'Net::IPTrie::_Node::up'};
 	}
     }