I turned off autovivification in my code, but for loops broke that and my hashes got populated with unwanted data.

For loops do aliasing, and array is created to be aliased... But I really don't understand my code works despite reading documentation. Could anyone explain why @{$$n{'x'}} throws exception outside loop, and is accepted inside loop (see code below), and why square brackets make so much difference to autovivification?

#!/usr/bin/perl -CSDA

use utf8; 
use strict; 
use warnings; 
no warnings qw{uninitialized numeric}; 
no autovivification; #  qw{fetch exists delete store}; 
use Data::Dumper; 

print "\n\ninitialize data structures, no autovivification\n"; 
my $n = { 
    a => [1, 2 ], 
    b => [3, 4], 
}; 
$$n{'x'}{'y'}{'z'} and die; 
$$n{'x'}{'y'}[0] and die; 
$$n{'x'}{'y'} and die; 
$$n{'x'} and die; 
print "so far, so good, no 'x' autovivified:\n"; 
print Dumper $n; 

# "Can't use an undefined value as an ARRAY reference": 
# @{$$n{'x'}} and die;  

print "\n\nnow the same but in loop:\n"; 
for my $e (@{$$n{'x'}}) { 
}
print "why 'x' autovivified?\n"; 
print Dumper $n; 

print "\n\nnow the same but in loop:\n"; 
for my $e (@{[ $$n{'y'} ]}) { 
}
print "why 'y' NOT autovivified?\n"; 
print Dumper $n; 


1;
[download]

Here comes the output of this program:

initialize data structures, no autovivification
so far, so good, no 'x' autovivified:
$VAR1 = {
          'a' => [
                   1,
                   2
                 ],
          'b' => [
                   3,
                   4
                 ]
        };


now the same but in loop:
why 'x' autovivified?
$VAR1 = {
          'x' => [],
          'a' => [
                   1,
                   2
                 ],
          'b' => [
                   3,
                   4
                 ]
        };


now the same but in loop:
why 'y' NOT autovivified?
$VAR1 = {
          'x' => [],
          'a' => [
                   1,
                   2
                 ],
          'b' => [
                   3,
                   4
                 ]
        };
[download]

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