use strict;
use warnings;

my $pid = open my $CMD, "| sar -c 1 2 \&>/dev/null" or die $!;

while (1) {
   open my $STAT,'<',"/proc/$pid/stat" or die $!;
   my $state = (split /\s+/, <$STAT>)[2];
   close $STAT;

   if ($state eq 'Z') {
      close $CMD or die $!;
      waitpid($pid,0);
      last;
   }

   sleep 1;
}

$ cat forked.pl
#!/usr/bin/perl
use strict;
use warnings;
open FH, ">forked.dat" or die $!;
print FH "forked $$\n";
close FH;
print STDERR "This should not appear ($$)\n";

$ perl -wle'
if (my $pid = fork()) {
  print "parent $$ forked $pid";
}
elsif (defined $pid) {
  close STDERR;
  exec qw(perl forked.pl);
}
'
parent 29747 forked 29748

$ cat forked.dat 
forked 29748

$proc = Proc::Simple->new();
$status = $proc->start("/bin/echo", "hello", "world");
$pid = $proc->pid;

...
elsif (defined $pid) {
  close STDERR;

open STDOUT,'>','/dev/null';
open STDERR,'>','/dev/null';

use strict;
use warnings;

# die Prozess-IDs habe ich jetzt mal beispielhaft angelegt
my $self     = {};
$self->{23}  = {};
$self->{64}  = {};
$self->{745} = {};
my @pids     = ();

# 1. Beispiel
@pids = keys %$self;
while (@pids) {
   for my $pid (@pids) {
      @pids = grep { !/^$pid$/ } @pids unless -e "/proc/$pid";
   }
}

# 2. Beispiel
@pids = keys %$self;
while (@pids) {
   for my $i (reverse 0..$#pids) {
      splice(@pids, $i, 1) unless -e "/proc/$pids[$i]";
   }
}

# 3. Beispiel
$self->{$_}->{run} = 1 for keys %$self;
my $count = keys %$self;
while ($count) {
   for my $pid (sort keys %$self) {
      next unless $self->{$pid}->{run};
      unless (-e "/proc/$pid") {
         $count--;
         $self->{$pid}->{run} = 0;
      }
   }
}