#!/usr/bin/perl

sub pipeline {
  pipe my ($read, $write);
  my $pid = open(my $fh, "-|");
  die "Couldn't fork: $!" unless defined $pid;
  if ($pid) {
    close $write;
    chomp(my @pids = map { $_->[1] } sort { $b->[0] <=> $a->[0] } map { [ split /_/ ] } <$read>);
    close $read;
    return $fh, @pids;
  }
  close $read;
  exit unless @_;
  my $cnt;
  while (my $command = pop) {
    $cnt++;
    my $pid = @_ ? open(STDIN, "-|") : -1;
    die "Couldn't fork: $!" unless defined $pid;
    next unless $pid;
    print $write "${cnt}_$$\n";
    close $write;
    exec @$command;
    die "Couldn't exec @$command";
  }
}

# Silly example, but you get the idea
# essentially the same as:
# open($fh, "ls -1 | sed -e s/t/a/g |")
my ($fh, @pids) = pipeline(
  [qw(ls -1)],
  [qw(sed -e s/t/a/g)],
);
while (<$fh>) {
  print "$.:$_";
}
close $fh;

print "Pids: @pids\n";