Let's break this up a bit, remembering that assignment is right-associative:

  $c = 2; # Assign 2 to $c
  $d = 3; # Assign 3 to $d

  [$c, $d] = [1, 2]; # Assign 1 to $c, assign 2 to $d

  $f = [$c, $d]; # Assign [1, 2] to $f