You're partly correct. In the case of this example, where there is only a single level hash, exists will not autovivify the key. However, if this was a complex data structure containing a HoH, HoA, AoH, etc, then the intermediate references (but not the terminal reference) would be autovivified*. To avoid autovivifying the intermediate structures you have to use exists to check each one as you go, or use something like tye's Data::Diver. See the example below. 

use strict;
use warnings;

use Data::Diver qw( Dive );
use Data::Dumper;

my %h;

# The exists function will autovivify intermediate keys

print "Testing with exists\n";
printf "key1 %s exist\n", exists $h{key1} ? 'does' : 'does not';
printf "key1-key2 %s exist\n", exists $h{key1}{key2} ? 'does' : 'does 
+not';
printf "key1 %s exist\n", exists $h{key1} ? 'does' : 'does not';
print Dumper( \%h );

# Data::Diver's Dive will not autovivify intermediate keys

print "\nTesting with Data::Diver\n";
my @values = Dive ( \%h, 'key3', 'key4' );
printf "key3-key4 %s exist\n", @values ? 'does' : 'does not';
print Dumper( \%h );

[download]
Output: 
Testing with exists
key1 does not exist
key1-key2 does not exist
key1 does exist
$VAR1 = {
          'key1' => {}
        };

Testing with Data::Diver
key3-key4 does not exist
$VAR1 = {
          'key1' => {}
        };

[download]

*You may know this, but someone else reading your reply may not realize the difference in behavior for single level vs multi level structures.

In reply to Re^3: What protects me from doing this stupid thing..? (autovivification) by bobf
in thread What protects me from doing this stupid thing..? by Cody Pendant

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