Sorry, as usually I forgot that people don't know the context of my question. I do know how to use perl and perl modules. I just was hoping that someone knows how to use specific feature of MongoDB via perl module. I guess this is wrong place to ask such questions. It would be better to contact some mailing list of MongoDB module users.
Anyway, it looks like I've found the answer myself. When connecting to MongoDB one can specify WriteConcern using attributes w, j, and wtimeout. This is described in perldoc MongoDB::MongoClient. So the connecting code would look like
my $cnx = MongoDB::MongoClient->new(
host => 'mongodb://myhost1,myhost2',
w => 'majority',
j => 1,
wtimeout => 10000,
replicaSet => 'myset');
At least this way after killing primary I don't loose data anymore.