UPDATE:
the code doesn't work, the problem was that <CHILD> reads upon \n and with a prompt, that could be a problem.

too hasty post:
i've been messing around in your code and this seems to work:

use Socket;
use IO::Handle;
use POSIX ":sys_wait_h";

my $childPid;

socketpair(CHILD, PARENT, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
CHILD->autoflush(1);
PARENT->autoflush(1);

die "no child fork" unless defined($childPid=fork());
if ($childPid){
  shutdown(PARENT,0);
  print "about to read\n";
  while (<CHILD>){
    last if (/Are you ready\? > /);
    print "got $_";
  }

  print PARENT "yes\n";
  print "we're ready\n";
}
else {
print "waiting for pid..\n";
}
open(STDOUT, ">&",PARENT) or die "Can't dup stdout: $!\n";
open(STDIN, "<&",PARENT) or die "Can't dup stdin: $!\n";
exec "./fill.pl" or die "Can't start my program: $!\n";

shutdown(CHILD,1);
[download]

and i used this as the fill.pl:

#!/usr/bin/perl

print <<END;
bla bla
bla
bla
bla
bla
lb
alb
blla bla
bla
bla bla bla
END

ASK: print "Are you ready? > \n";
$a=<STDIN>;
if ($a =~ /yes/) {print "bla bla bla";exit 0;}
else {goto ASK;}
[download]

hope this helps