#!/usr/bin/perl

use warnings;
use strict;

use IO::Select;

# Number of simultaneous processes
my $n = 5;

# List of things to process
my @p = (1..20);

my $sel = IO::Select->new();
for my $pr (splice @p, 0, $n) {
  start_ps($sel, $pr);
}

my $cnt = 1;
while ( my @ready = $sel->can_read() ) {
  for my $h (@ready) {
    local $_;
    my ($fh, $i) = @$h;
    while (<$fh>) {
      print "$i: $_";
    }
    # Remove, then close!!!
    $sel->remove($fh);
    close $fh;
    start_ps($sel, shift(@p)) if @p;
    $cnt++;
  }
  print "-------------\n";
}

sub start_ps {
  my ($t, $p) = @_;
  my $pid = open(my $fh, "-|");
  die "Can't fork: $!" unless defined $pid;
  unless ($pid) {
    my @cmd = qw(/bin/echo Hello World);
    exec @cmd;
    die "Could not exec @cmd: $!";
  }
  $t->add([$fh, $p]);
}