sub new {
    my ($class, %arg) = @_;
# ...
    bless {
# ...
        _players => $arg{players} ,
    }, $class;
}

sub play {
    my ($self) = @_;
    my %player;
    $self->{_players}[0]->out();  # this works
    # out() is a member function that prints some info about the Player

    # add key containg ref to 1st player to %player and
    # with value of a reference to a new anonymous hash
    $player{$self->{_players}[0]} = {};
    foreach my $this_player (keys %player) {
        $this_player->out(); # this doesn't
    }
}