sub myWrite {
    my( $port, $data ) = @_;
    for my $chunk ( unpack '(a15)*', $data ) {
        $port->write( $chunk );
        sleep 0.1; ## may not be necessary; or maybe 0.001 is enough; experiment.
    }
}

...
# $ob->write( "$string\n");
myWrite( $ob, $string );