attribute not being set...

gistfile1.pl

class SudokuSolver {
     has Int @!grid is rw;   

     method build(Str $s) {
         @!grid = $s.comb();
     }

     method solve() {
         self.placeNumber(0);
     }

     method placeNumber(Int $pos) {
        if ($pos == 81) {
            say 'final placeNumber and returning';
            
            self.toString().say;
            
            @!grid[0]=5;
            return True;
        }

        
        if (@!grid[$pos] > 0) {
            self.placeNumber($pos + 1);
            return True;
        }


        for (1..9) -> $n {
            if (self.checkValidity($n, $pos % 9, ($pos / 9).Int) ) {
                 @!grid[$pos] = $n;
                 self.placeNumber($pos + 1);
                 @!grid[$pos] = 0;
            }

        }
        
         
         return True;
     } 


    method checkValidity(Int $val, Int $x, Int $y) {
        for ^9 -> $i {
            if (@!grid[$y * 9 + $i] == $val || @!grid[$i * 9 + $x] == $val) {
                return False;
            }
        }
        
        my Int $startX = ($x / 3).Int * 3;
        my Int $startY = ($y / 3).Int * 3;
        
        loop (my Int $a = $startY; $a < $startY + 3; $a++) {
             loop (my Int $j = $startX; $j < $startX + 3; $j++) {
                 if (@!grid[$a * 9 + $j] == $val) {
                     return False;
                 }
                 
             }
        }
        
        return True;
    }

     
method toString() {
    my Str $sb;

    for ^9 -> $i {
        for ^9 -> $j {
            $sb ~= @!grid[$i * 9 + $j] ~ ' ';
            if ($j == 2 | 5) {
                $sb ~="| ";
            }
            
        }
        $sb ~= "\n";
        if ($i == 2 | 5) {
            $sb ~= "------+-------+------\n";
        }
        
    }
    return $sb;
}

     
}
    my $x =SudokuSolver.new();


$x.build('034167598' ~
             '198543627' ~
                 '765982431' ~
                     '987036215' ~
                         '653219874' ~
                             '421758069' ~
                                 '819625743' ~
                                     '542371986' ~
                                         '376894150');

$x.toString().say;

$x.solve();

say 'outside placeNumber';

$x.toString().say;