What exactly do you mean by "I don't want to build a listener in the parent." Clearly the parent has got to listen in some way. There are couple of ways to do non-network IPC. You can use basic pipes:

pipe(README, WRITEME);
if (fork) {
  #parent code
  while (<README>) {
     # you can do other stuff in here besides reading
     $output .= $_;
  }
  close(README);
} else {
  # child code
  print WRITEME "interesting stuff";
  close(WRITEME);
}
[download]

There's also named pipes, which are nice in cases when there isn't a relationship between the communicating processes (although you could use them in this case if you want).

# writer
open(FIFO, "> /path/to/named.pipe");
print FIFO "Interesting stuff";
close(FIFO);

#reader
open(FIFO, "< /path/to/named.pipe");
while(<FIFO>) {
  $output .= $_;
}
close(FIFO);
[download]

With named pipes, readers and writers will block until their counterpart appears at the other side unless you open the fifo using +<. All of these examples are out of the Perl Cookbook, chapter 16.

Update: Forgot to mention you have to create the named fifo first, from the shell. On most systems this is

mkfifo /path/to/named.pipe

Older systems use mknod.

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