DB<2> $mref = [ qw / 5 4 4 2 4/ ];

  DB<3> x \$mref
0  REF(0x600500ac8)
   -> ARRAY(0x600509918)
         0  5
         1  4
         2  4
         3  2
         4  4

  DB<4> $nref = [ map {[$_]} @$mref ];

  DB<5> x \$nref
0  REF(0x600500708)
   -> ARRAY(0x6005e8c88)
         0  ARRAY(0x6005ea970)
            0  5
         1  ARRAY(0x6005f3d20)
            0  4
         2  ARRAY(0x6004bdfd0)
            0  4
         3  ARRAY(0x60060d270)
            0  2
         4  ARRAY(0x6005e8c40)
            0  4