add on top of what sporty quoted, exists() is just one case for autovivification. To be precise on autovivification: any attempt to read or test an element that does not exist, will create its ancestors.

use Data::Dumper;
use strict;
use warnings;

my $data = {"a" => 1};
$data->{"b"}->{"b"}->{"b"}->{"b"};#useless use of hash element in void
+ context? interesting error message. The useless use is actually not 
+that useless.
print Dumper($data);
[download]

this prints:

$VAR1 = {
          'a' => 1,
          'b' => {
                   'b' => {
                            'b' => {}
                          }
                 }
        };
[download]