Assuming that you want to wait on a subroutine to finish that has system commands (or even just parts of the script that you may like to run in parallel), take a look at Parallel::ForkManager. Put whatever wait code you want in the run_on_wait() sub. Everything else goes into the foreach loop.

use Parallel::ForkManager;

 # Begin ForkManager
 my $_max_procs = 5;
 my $_pm = new Parallel::ForkManager($_max_procs);

 # Log at process fork
 $_pm ->run_on_start(
   sub { my ($pid, $host) = @_;
     print "Forking process PID: $pid\n";
   }
 );

 # Log at process copmletion
 $_pm ->run_on_finish(
   sub { my ($pid, $exit_code, $func) = @_;
     print "Finishing up process PID: $pid\n";
   }
 );

 # This run_on_wait is currently set to print every 5 sec
 # It can easily be modified to write to a socket every X sec
 $_pm->run_on_wait(
   sub { print "Waiting for children to finish.\n" }, 5.0 );

foreach my $cmd (@commands) {
  # Fork off the children
  my $pid = $_pm->start($cmd) and next;

  # run subroutine to wait on here

  # Closing the forked process
  $_pm->finish;
}

# Ensure all children have finished
$_pm->wait_all_children;
exit(1);
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