I don't get either "Oh No!" comments.

Then $rec{NOTE} exists, but contains a value other than "Beware!".

But with Nested Hashes I still get the intermediate hash autovivified

The following contains a dereference: 

$rec{NOTE}{Nested}

[download]

You could have written it as follows: 

$rec{NOTE}->{Nested}

[download]

If you factor in autovivification, then the above is equivalent to the following: 

( $rec{NOTE} //= {} )->{Nested}

[download]

To get autovivified, $rec{NOTE} has to be undefined (or nonexistent), which is consistent with what we already knew (existent and not equal to "Beware!").

By the way, that means (1) would have given you a warning if you had warnings on as you should. 

use strict;
use warnings;
no warnings 'void';

my %rec;

printf "exists:  %s\n", exists($rec{NOTE})  ?1:0; 
printf "defined: %s\n", defined($rec{NOTE}) ?1:0;
printf "true:    %s\n", $rec{NOTE}          ?1:0;
print "--\n";

$rec{NOTE} = undef;

printf "exists:  %s\n", exists($rec{NOTE})  ?1:0; 
printf "defined: %s\n", defined($rec{NOTE}) ?1:0;
printf "true:    %s\n", $rec{NOTE}          ?1:0;
print "--\n";

$rec{NOTE}{Nested};

printf "exists:  %s\n", exists($rec{NOTE})  ?1:0; 
printf "defined: %s\n", defined($rec{NOTE}) ?1:0;
printf "true:    %s\n", $rec{NOTE}          ?1:0;

[download]
exists:  0
defined: 0
true:    0
--
exists:  1
defined: 0
true:    0
--
exists:  1
defined: 1
true:    1

[download]

In reply to Re: gotchas with hash element autovivification by ikegami
in thread gotchas with hash element autovivification by raybies

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.