You didn't change $r the accessed value in your snippet — you changed that which is pointed by $r — so you failed to disprove that $r the accessed value is read-only.

Say you want to do $my_hash_ref->{$k[0]}->{$k[1]}->{$k[2]} = 3;. Compare 

use Data::Dumper;

my $my_hash_ref = { a => { b => { c => 2 } } };

my @k = qw( a b c );

my $r = $my_hash_ref;
$r = $r->{$_} foreach @k;
$r = 3;

print Dumper($my_hash_ref);
# { 'a' => { 'b' => { 'c' => 2 } } };

[download]

with 

use Data::Dumper;

my $my_hash_ref = { a => { b => { c => 2 } } };

my @k = qw( a b c );

my $p = \$my_hash_ref;
$p = \($$p->{$_}) foreach @k;
$$p = 3;

print Dumper($my_hash_ref);
# { 'a' => { 'b' => { 'c' => 3 } } };

[download]

Update: Fixed a terminology error.

In reply to Re^3: I have a hash in a hash in a ..... by ikegami
in thread I have a hash in a hash in a ..... by Anonymous Monk

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