my %subs = (
  PR1 = \&process1,
  PR2 = \&process2,
  PR3 = \&process3,
  PR4 = \&process4,
);

##</code><code>##

my @processes;

foreach (sort { $po{$a} <=> $po{b} } keys %po) {
  push @processes, $_;
}

##</code><code>##

foreach (sort { $po{$a} <=> $po{b} } keys %po) {
  push @processes, $subs{$_};
}

##</code><code>##

foreach (@processes) {
  $input = $_->($input);
}