undef $old;

$old = {
         'child' => {
                      'parent' => $old,  # !!!
                      'name' => 'bob jr'
                    },
         'name' => 'bob'
       };

##</code><code>##

#!/usr/bin/perl -w
use strict;
use Data::Dumper;

my $old = {
	name => "bob",
	child => {
		name => "bob jr",
	}
};
$old->{child}{parent} = $old;

$Data::Dumper::Purity = 1;
print Data::Dumper->Dump([$old], ['$old']);

__END__
$old = {
         'child' => {
                      'parent' => {},
                      'name' => 'bob jr'
                    },
         'name' => 'bob'
       };
$old->{'child'}{'parent'} = $old;