open DATE, "date|" or die "cannot pipe from date: $!";
open MAIL, "|mail merlyn" or die "cannot pipe to mail: $!";

The open fails if the child process cannot be created. If the command itself does not exist or exits erroneously, this will (generally) not be seen as an error when opening, but as an error when closing. We'll get to that in a moment.

For all intents and purposes, the rest of the program doesn't know, doesn't care, and would have to work pretty hard to figure out that this is a filehandle opened on a process rather than on a file. So, to get data from a filehandle opened for reading, we'll just do the normal read:

my $now = <DATE>;

And to send data to the mail process (waiting for the body of a message to deliver to merlyn on standard input), a simple print-with-a-filehandle will do:

print MAIL "The time is now $now"; # presume $now ends in newline

In short, you can pretend that these filehandles are hooked up to magical files, one that contains the output of the date command, and one that will automatically be mailed by the mail command.

If a process is connected to a filehandle that is open for reading, and then exits, the filehandle returns end-of-file, just like reading up to the end of a normal file. When you close a filehandle open for writing to a process, the process will see end-of-file. So, to finish sending the email, close the handle:

close MAIL;
die "mail: non-zero exit of $?" if $?;

Closing a filehandle attached to a process waits for the process to complete, so that Perl can get the process's exit status. The exit status is then available in the $? variable (reminiscent of the same variable in the Bourne Shell), and is the same kind of number as the value returned by the system function: zero for success, nonzero for failure. Each new exited process overwrites the previous value though, so save it quickly if you want it. (The $? variable also holds the exit status of the most recent system or backquoted command, if you're curious.)

The processes are synchronized just like a pipelined command. If you try to read and no data is available, the process is suspended (without consuming additional CPU time) until the sending program has started speaking again. Similarly, if a writing process gets ahead of the reading process, the writing process is slowed down until the reader starts to catch up. There's a buffer (usually 4K bytes or so) in between so they don't have to stay precisely in lock step.

Why use processes as filehandles? Well, it's the only easy way to write to a process based on the results of a computation. But if you're just reading, backquotes are often much easier to manage, unless you want to have the results as they come in.

open F, "find / -atime +90 -size +1000 -print|" or die "fork: $!";
while (<F>) {
  chomp;
  printf "%s size %dK last accessed on %s\n",
    $_, (1023 + -s $_)/1024, -A $_;
}