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  my @sparse = () ;
  while (...whatever...) {
...
  } ;

  @sparse = sort { ($$a[0] <=> $$b[0]) || ($$a[1] <=> $$b[1]) || die }
+ @sparse ;

use strict ;
use warnings ;
...
       0  -1  -3   2   0
       0   0   1   0   0
       0   4   2   0   1