my @sorted = sort {$po{$a} <=> $po{$b}} keys %po;

##</code><code>##

my %process = (
    PR1 => sub { $_[0] = process1($_[0]) },
    PR2 => sub { $_[0] = process2($_[0]) },
    # . . .
);

my %po = (
    PR1 => 4,
    PR2 => 2,
    PR3 => 1,
    PR4 => 3,
    PR5 => 5,
);

for (sort {$po{$a} <=> $po{$b}} keys %po) {
    $process{$_}($input);
}