my $client = Device::Modbus::RTU::Client->new(
    port     => '/dev/ttyUSB0',
    baudrate => 4800,
    parity   => 'none',
);
 
my $req = $client->read_holding_registers(
    address  => 1, # -r
    quantity => 1, #-c
);
 
$client->send_request($req);
my $resp = $client->receive_response;