sub sms_2x2_PDL_wlmb ( $m ) {
    pdl( 
        $m-> slice( '0:-2,0:-2' ),
        $m-> slice( '1:-1,0:-2' ),
        $m-> slice( '0:-2,1:-1' ),
        $m-> slice( '1:-1,1:-1' ),
    )-> mv( -1, 0 )-> sumover
}

__END__

               Time (s) vs. N (2x2 submatrix, NxN matrix)

      +-----------------------------------------------------------+
      |+          +         +          +          +          +    |
      |                                                      B    |
      |                                                           |
      |                                                           |
      |                                                           |
  1.5 |-+                                                       +-|
      |                                                           |
      |                                                           |
      |                                                           |
      |                                           B               |
      |                                                           |
    1 |-+                                                       +-|
      |                                                           |
      |                                                           |
      |                                                           |
      |                                                      A    |
      |                                                           |
      |                                B                          |
  0.5 |-+                                                       +-|
      |                                           A               |
      |                                                           |
      |                     B                                     |
      |                 B              A                          |
      |             B       A                                     |
      |        B    A   A                                         |
    0 |-+  B   A                                                +-|
      |+          +         +          +          +          +    |
      +-----------------------------------------------------------+
       0         1000      2000       3000       4000       5000
                             sms_2x2_PDL    A
                        sms_2x2_PDL_wlmb    B


  +------+-------+-------+
  | N    | A     | B     |
  +------+-------+-------+
  | 400  | 0.002 | 0.011 |
  | 800  | 0.006 | 0.041 |
  | 1200 | 0.023 | 0.098 |
  | 1600 | 0.061 | 0.180 |
  | 2000 | 0.109 | 0.273 |
  | 3000 | 0.238 | 0.613 |
  | 4000 | 0.433 | 1.148 |
  | 5000 | 0.738 | 1.758 |
  +------+-------+-------+