my @sparse = () ;
  while (...whatever...) {
    my $value = ... ;  # next value
    my $col   = ... ;  # in this column
    my $row   = ... ;  # and this row
    if ($value) {      # keep only the non-zeros
      push @sparse, [$col, $row, $value] ;
    } ;
  } ;

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