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sub up
  {
...
    ($row * $col > 0 && up( ~-$row . '-' . ~-$col, @_ ) ) .
      ($row > $col && up( ~-$row . '-' . $col, @_ ) );
  }

sub up{
        # indent to evidentiate the recursive function level
...
            ($row > $col && up( ~-$row . '-' . $col, @_ ) );
        } 
}

@_ = [3-1]
3 * 1 > 0 && up(2-0, 3-1) # decremented both row and column are passed
+ plus original @_
...
            RETURNING: @_ is a valid path [0-0 1-1 2-1 3-1]
        1 > 1 is FALSE...
"0-0 1-0 2-0 3-1\n0-0 1-0 2-1 3-1\n0-0 1-1 2-1 3-1\n"

# receives and returns aoa
 sub up_modified{
...
        );
    }    
}

@_ is [[3, 1]]
receiving row 3 col 1
...
            @_ is [[0, 0], [1, 1], [2, 1], [3, 1]]
            receiving row 0 col 0
            RETURNING: [[0, 0], [1, 1], [2, 1], [3, 1]]

 sub up_modified_bis{
    my $ind = ' ' x $#_ x 4;
...
            @_ is ([0, 0], [1, 1], [2, 1], [3, 1])
            receiving row 0 col 0
            RETURNING: ([0, 0], [1, 1], [2, 1], [3, 1])