How do I read and write the serial port?

This depends on which operating system your program is running on. In the case of Unix, the serial ports will be accessible through files in /dev; on other systems, the devices names will doubtless differ. Several problem areas common to all device interaction are the following

lockfiles
Your system may use lockfiles to control multiple access. Make sure you follow the correct protocol. Unpredictable behaviour can result from multiple processes reading from one device.

open mode
If you expect to use both read and write operations on the device, you'll have to open it for update (see open for details). You may wish to open it without running the risk of blocking by using sysopen and O_RDWR|O_NDELAY|O_NOCTTY from the Fcntl module (part of the standard perl distribution). See sysopen for more on this approach.

end of line
Some devices will be expecting a ``\r'' at the end of each line rather than a ``\n''. In some ports of perl, ``\r'' and ``\n'' are different from their usual (Unix) ASCII values of ``\012'' and ``\015''. You may have to give the numeric values you want directly, using octal (``\015''), hex (``0x0D''), or as a control-character specification (``\cM''). 

    print DEV "atv1\012";	# wrong, for some devices
    print DEV "atv1\015";	# right, for some devices

Even though with normal text files, a ``\n'' will do the trick, over the network, there is still no unified scheme for terminating a line that is cross-platform to all of Unix, DOS/Win, and Macintosh, except to terminate ALL line ends with ``\015\012'', and strip what you don't need from the output. This applies especially to socket I/O and autoflushing, discussed next.

flushing output
If you expect characters to get to your device when you print them, you'll want to autoflush that filehandle, as in the older 

    use FileHandle;
    DEV->autoflush(1);

and the newer 

    use IO::Handle;
    DEV->autoflush(1);

You can use select and the $| variable to control autoflushing (see $| and select): 

    $oldh = select(DEV);
    $| = 1;
    select($oldh);

You'll also see code that does this without a temporary variable, as in 

    select((select(DEV), $| = 1)[0]);

As mentioned in the previous item, this still doesn't work when using socket I/O between Unix and Macintosh. You'll need to hardcode your line terminators, in that case.

non-blocking input
If you are doing a blocking read or sysread, you'll have to arrange for an alarm handler to provide a timeout (see alarm). If you have a non-blocking open, you'll likely have a non-blocking read, which means you may have to use a 4-arg select to determine whether I/O is ready on that device (see select.