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;
      }
   }
}