No, it is because in effect, you execute the following: 
undef $old;

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

[download]

Obviously, the parent key is assigned the new value of $old, which is, well, undef.

Update: You want to check the Data::Dumper POD for the $Data::Dumper::Purity setting.

#!/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;

[download]
Update2: it appears valdez snuck the same point in as I was looking up the POD myself.

Makeshifts last the longest.

In reply to Re: evaling dumped cyclic structures by Aristotle
in thread evaling dumped cyclic structures by marvell

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.