perl process queue

Check out Parallel::ForkManager -- it does much of what you describe. You can set a maximum number of processes, and the callback function could start a new child as soon as one finishes (as long as there is work to do).

While I would almost always use a CPAN module, or write something with the fantastic AnyEvent modules I think its important to understand how these things work under the hood. Here's an example that has no dependencies other than perl. The same approach could also be written in C without too much trouble.

#!/usr/bin/env perl

use strict;

## run a function in a forked process
sub background (&) {
  my $code = shift;

  my $pid = fork;
  if ($pid) {
    return $pid;
  } elsif ($pid == 0) {
    $code->();
    exit;
  } else{
    die "cant fork: $!"
  }
}

my @work = ('sleep 30') x 8;
my %pids = ();
for (1..4) {
  my $w = shift @work;
  my $pid = background {
    exec $w;
  };
  $pids{$pid} = $w; 
}

while (my $pid = waitpid(-1,0)) {
  if ($?) {
    if ($? & 127) {
      warn "child died with signal " . ($? & 127);
    } else {
      warn "chiled exited with value " . ($? >> 8);
    }

    ## redo work that died or got killed
    my $npid = background {
      exec $pids{$pid};
    };
    $pids{$npid} = delete $pids{$pid};
  } else {
    delete $pids{$pid};

    ## send more work if there is any
    if (my $w = shift @work) {
      my $pid = background {
        exec shift @work;
      };
      $pids{$pid} = $w;
    }
  }
}