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

my @processes;

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

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

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