sub DiveVal :lvalue {
   my $p = \shift;
   $p = \( $$p->{$_} ) for @_;
   $$p
}

my @keys = map "level$_", 1 .. 3;

my $branch;
DiveVal($branch, @keys) = 'leaf';
  -or-
my %branch;
DiveVal(\%branch, @keys) = 'leaf';