use File::Slurp qw( append_file );

if ($foo) {
    append_file('/dev/ttyACM0',{ err_mode => 'croak', binmode => ':raw' },"\xFF\x08\x00");
} else {
    append_file('/dev/ttyACM0',{ err_mode => 'croak', binmode => ':raw' },"\xFF\x08\x01");
}