my @k = qw( a b c );

$my_hash_ref->{$k[0]}->{$k[1]}->{$k[2]} = 2; 

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

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