Some problem with Perl driver and aggregation

[Roman Rakhman]

Hi, mongodb-users.

I have collection 'books': 

{

    "_id" : ObjectId("54901b078b823f2014080700"),

    "title" : "title1",

    "available" : 0,

    "time" : 14318789245,
    "score" : 1,

   "categories" : [ 

        "fiction", 

        "humor"

    ]

},

{

    "_id" : ObjectId("54901b078b823f2014080701"),

    "title" : "title2",

    "available" : 1,

    "time" : 14318789745,
    "score" : 1,
    "score_fiction": 5,

    "categories" : [ 

        "fiction", 

    ]

}
...

{

    "_id" : ObjectId("54901b078b823f2014080751"),

    "title" : "titleN",

    "available" : 1,

    "time" : 14318789545,
    "score" : 4,
    "score_fiction": 2,

}

I will aggregate it with this script:

my $books = $mongo->db('test')->get_collection('books')->aggregate(

    [

    {'$match' => {categories => fiction}},

    {'$project' => {

                 'av' => {'$cmp' => ['$available', 0] },

                 title => 1,

                 scr => {'$ifNull' => ['$score_fiction' , '$score'] },

                 time =>1}

    },

    {'$sort' => {av => -1, scr => -1, time => -1}},

    {'$limit' =>  10}

    ], {cursor => 1}

);

while (my $book = $books->next) {

print $book->{title} . " [" . $book->{scr} . "]\n";

After 5-7 runs I get two different results: 

1. With sorting by required fields. 

2. Without sorting. (possibly with sorting by time or the "$natural" time of addition).

When I run this aggregation pipeline in mongo's shell - it works with correctly sorting.

db.books.aggregate([{'$match' : {categories : 'fiction'},
{'$project' : {'av' : {'$cmp' : ['$available', 0] },
  title : 1,
  scr : {'$ifNull' : ['$score_fiction' , '$score'] },
  time:1}},
{'$sort' : {av : -1,scr :-1, time : -1}},
{'$limit':  10}]);

Is the problem in the driver or in my head?
PS
Perl driver MongoDB v0.708.1.0 
Mongo v 3.0.3