DB<107> @a = (1..5)                             # @array
 => (1, 2, 3, 4, 5)

  DB<108> $#a                                     # classic usage
 => 4

  DB<109> $a = [1..10]                            # arrref
 => [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

  DB<110> $#$a
 => 9

  DB<111> $a = [ map [0..$_],0..3 ]               # AoA
 => [[0], [0, 1], [0, 1, 2], [0, 1, 2, 3]]

  DB<112> $#{ $a->[2] }
 => 2