# %nested_hash revised to contain a circularity
my $hAA = {aaa => 1, aab => 2 };
$hAA->{aac} = $hAA;

my %nested_hash = ( a => { aa => $hAA,
                           ab => {aba => 4,
                                  abb => 5,
                                  abc => 6} },
                    b => { ba => {baa => 7,
                                  bab => 8,
                                  bac => 9},
                           bb => {bba => 10,
                                  bbb => 11,
                                  bbc => 12} } );


sub walk {
  my $node = shift;

 OUTER:
  foreach my $k (sort keys %{ $node }) {

    # replace these lines with the lines between ***
    # push @nodes, $k;
    # print join(", ", @nodes), "\n";
    # ***

    print join(", ", @nodes) . (scalar(@nodes)?', ':'');
    foreach my $kInPath (@nodes) {
      if ($k eq $kInPath) {
        print "$k ... (circularity)\n";
        next OUTER if ($k eq $kInPath);
      }
    }

    print "$k\n";
    push @nodes, $k;

    # ***

    walk( $node->{$k} ) if ref $node->{$k};
    pop @nodes;
  }
}

# outputs
a
a, aa
a, aa, aaa
a, aa, aab
a, aa, aac
a, aa, aac, aaa
a, aa, aac, aab
a, aa, aac, aac ... (circularity)
a, ab
a, ab, aba
a, ab, abb
a, ab, abc
b
b, ba
b, ba, baa
b, ba, bab
b, ba, bac
b, bb
b, bb, bba
b, bb, bbb
b, bb, bbc