I would create a set of pipes, using IO::Pipe between the parent, and the child, and have the child send immediately to the parent with autoflush turned on.

Here is a simple example( not using IO::Pipe ).

#!/usr/bin/perl
use warnings;
use strict;
use IO::Handle;

$|++;
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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