use IO::Handle;

use strict;

$| ++;
pipe(PARENT_READER, CHILD_WRITER);
pipe(CHILD_READER, PARENT_WRITER);
PARENT_WRITER->autoflush(1);
CHILD_WRITER->autoflush(1);

my $pid;
if ($pid = fork()) {
    close(CHILD_READER);
    close(CHILD_WRITER);
    my $buffer;
    print PARENT_WRITER 1;
    while (1) {
        sysread(PARENT_READER, $buffer, 100);
        print "parent revd: $buffer, and reply with ", $buffer + 1, "\
+n";
        sleep(1);
        print PARENT_WRITER $buffer + 1;
    }
} else {
    close(PARENT_READER);
    close(PARENT_WRITER);
    my $buffer;
    while (1) {
        sysread(CHILD_READER, $buffer, 1000);
        print "child revd: $buffer, and reply with ", $buffer + 1, "\n
+";
        sleep(1);
        print CHILD_WRITER $buffer + 1;
    }
}
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I wrote a filter to act as a sniffer for a specific network protocol. I wrote code in Win32 to use IO completion ports to efficiently move the buffer from one to the other without copying it, and also passed it to an embedded Perl interpreter as a "tee" in the middle. The Perl could do its fine job of parsing the stuff and presenting it to me, but the processes piped efficiently as long as I didn't select the option for modification of the data stream by the Perl code (that is, report only). I used the modification ability to introduce errors or otherwise test things that my well-behaved program never did.

—John