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use Data::Dumper;

...

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

use Data::Dumper;

...

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

my @k = qw( a b c );

...
$r = 3;

print($my_hash_ref->{$k[0]}->{$k[1]}->{$k[2]}, "\n");   # 2

my @k = qw( a b c );

...
$$p = 3;

print($my_hash_ref->{$k[0]}->{$k[1]}->{$k[2]}, "\n");   # 3