$k;         # == "1  136  G   A" with more spaces
   $r{$k};     # == [ [1,6], [1,9] ]  == referenece to an array of array-refs
   @{$r{$k}};  # == ( [1,6], [1,9] )  == array of array-refs
   map {} @    # for each element in the @ array, run the {} block

   # First element of @ is the first ref to a pair-array:
   $_;         # = [1,6] == array ref
   $$_;        # = (1,6) == array
   $$_[0];     # = 1 == first element of array (1,6) or arrayref [1,6]
   $$_[1];     # = 6

   # Second element of @ is the next ref to a pair-array
   $_;         # = [1,9]
   $$_[0];     # 1
   $$_[1];     # 9