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

...
            0  2
         4  ARRAY(0x6005e8c40)
            0  4

  DB<6> use Data::Dumper;

...
            ]
          ];

  DB<8> $mref = [  [ qw / 5 4 4 2 4/ ],  ];

  DB<9> $nref = [ map {[$_]} map {@{$_}} @$mref ];