Your first example is a hash slice but no hash was declared. In your second example, you declared the hash, but that's not autovivification. Autovivification is the automatic creation of a variable reference when an undefined value is dereferenced. Maybe this example will help:

#!/usr/bin/perl

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

my $hash = {
            'foo' => { 'abc' => 0, },
            'bar' => { 'def' => 1, },
};
print Dumper($hash) unless exists $hash->{'baz'}{'ghi'},
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'baz' doesn't exist, so perl creates it---autovivification.