Code-wise, I would make ->_set_connection respect the ->{port} and ->{host} arguments.

Also, I found a cascade of arguments makes testing and overriding things much easier in such routines. I would take the URL from the passed arguments, or from $self, in that order:

sub _set_connection {
    my( $self, %options ) = @_;
    my $url = $options{ url };
    $url ||= sprintf 'http://%s:%s', $self->{host}, $self->{port};
    return MongoDB->connect($url);
}
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