only add first_key for top-level documents
only add first_key for top-level documents

dagolden/mongo-perl-driver,gormanb/mongo-perl-driver,rahuldhodapkar/mongo-perl-driver,gormanb/mongo-perl-driver,kainwinterheart/mongo-perl-driver,xdg/mongo-perl-driver,kainwinterheart/mongo-perl-driver,jorol/mongo-perl-driver,dagolden/mongo-perl-driver,gormanb/mongo-perl-driver,rahuldhodapkar/mongo-perl-driver,dagolden/mongo-perl-driver,mongodb/mongo-perl-driver,mongodb/mongo-perl-driver,xdg/mongo-perl-driver,mongodb/mongo-perl-driver,jorol/mongo-perl-driver,dagolden/mongo-perl-driver,kainwinterheart/mongo-perl-driver,rahuldhodapkar/mongo-perl-driver,gormanb/mongo-perl-driver,jorol/mongo-perl-driver,jorol/mongo-perl-driver,rahuldhodapkar/mongo-perl-driver,kainwinterheart/mongo-perl-driver,xdg/mongo-perl-driver