my($app1_pid,$app1_out)=pipeline(STDIN,'app1',$URL);
my($app2_pid,$app2_out)=pipeline($app1_out,'app2');
while (<$app2_out>) { ... }

##</code><code>##

#!/usr/bin/perl

use POSIX;

my($pid1,$outfh1)=pipeline(STDIN,"sort");
my($pid2,$outfh2)=pipeline($outfh1,"uniq -c");
my($pid3,$outfh3)=pipeline($outfh2,"sort -rn");

warn "sort pid: $pid1\n";
warn "uniq pid: $pid2\n";
warn "sort2 pid: $pid3\n";

while (<$outfh3>)
{
  print;
}

sub pipeline
{
  my($in,@cmd)=@_;
  my($out,$child_write);
  pipe($out,$child_write)
    or die "pipe error: $!\n";
  my $pid = fork;
  if (!defined($pid)) { die "fork error: $!\n" };

  if ($pid)
  {
    # Parent
    close($child_write);
    return($pid,$out);
  }
  else
  {
    # child
    close($out);
    if (fileno($in) != 0)
    {
      close(STDIN)
        or die "Couldn't close STDIN: $!\n";
      POSIX::dup2(fileno($in),0)
        or die "Couldn't dup to stdin: $!\n";
    }
    close(STDOUT);
    POSIX::dup2(fileno($child_write),1)
      or die "Couldn't dup to stdout: $!\n";
    exec(@cmd)
      or die "exec error: $!\n";
  }
}