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$h = {
   'hello' => [
...
      [ 5, 6, 7, 8 ],
   ],
};

exists(  $h->{'hello'})       # checks if hello exists.
scalar(@{$h->{'hello'}})      # The number of lists (2).
         $h->{'hello'}[1]     # The second list ([5,6,7,8]).
scalar(@{$h->{'hello'}[1]})   # The size of the second list (4).
         $h->{'hello'}[1][0]  # The first num of second list (5).