use CGI qw(:all delete_all escapeHTML);

my $redirect_page = "display_page.cgi";

if (my $session = param('session')) { # returning to pick up session 
  #
  # ...(2) redirect
  #
  my $data = $cache->get($session);

  if ( $data->[0] ){
    #
    # ...(2a) flag set
    #
  }
  else{
    # ....
} 
elsif ( (my $slp = param('sleep')) eq 'sleep') {

    # ....(1) first invocation

    $cache->set($session, [0, ""]); # no data yet
    if (my $pid = fork) {       # parent does
      #
      # ....(1a) fork: parent
      #
      print redirect(self_url());
    } 
    elsif (defined $pid) {    # child does
      #
      # ....(1b) fork: child
      #
      $cache->set($session, [1, $buf]);
      exit 0;
    } 
    else {
      # ....
    }
}
else {                        # display form
  # ....
}

# ....