#!/usr/bin/perl

# http://perlmonks.org/?node_id=1137500

use IO::Select;
use YAML;
use strict;
use warnings;
$| = 1;

my $sel = IO::Select->new;
my (%hash, %id);
my @a = map "host$_", 1..9;
my $max = 4;

while(@a or $sel->count)
  {
  if(@a and $sel->count < $max)
    {
    my $host = shift @a;

    if(open my $fh, '-|')
      {
      $id{$fh} = $host;
      $sel->add($fh);
      $hash{$id{$fh}} .= "starting $host  ";
      print "running ", $sel->count, "\n";
      }
    else
      {
      print "hello from the $host child";
      exit;
      }
    }
  elsif($sel->count)
    {
    for my $fh ($sel->can_read)
      {
      if(0 < sysread $fh, my $buf, 1024)
        {
        $hash{$id{$fh}} .= $buf;
        }
      else
        {
        $sel->remove($fh);
        close $fh;
        }
      }
    }
  }

print Dump \%hash;
[download]

See, simpler with forked open instead of IO::Pipe when only reading from the child :)