for my $key ( keys %hash ) {
    for my $aref ( @{ $hash{ $key } } ) {
        @{ $aref } = sort { $b <=> $a } @{ $aref };
    }
}
;;

pp \%hash;;
{
  1 => [[28, 27, 26], [18, 17, 16], [8, 7, 6], [3, 2, 1]],
  2 => [[29, 28, 27], [19, 18, 17], [9, 8, 7], [4, 3, 2]],
  3 => [[30, 29, 28], [20, 19, 18], [10, 9, 8], [5, 4, 3]],
  4 => [[31, 30, 29], [21, 20, 19], [11, 10, 9], [6, 5, 4]],
}