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@a = ( [2,5,10,5,12,6,21,5,10,12,23],
       [5,6,11,10,5,10,6,21,5,1,9],
...
    );
$m = 2;
$n = 2;

(5,10) #which occured twice in 1st, once in 2nd and 3rd
(10,5) # which occured once in 1st and 2nd
(6,21,5) # which occured once in 1st and 2nd