You can sort the keys by value,

my @sorted = sort {$po{$a} <=> $po{$b}} keys %po;
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You would do better associating a name to coderef through a hash, an arrangement usually called a dispatch table. What you're trying is calling through symbolic references.

(Added) With your calling pattern of calling on and modifying $input, you might consider writing your subs to modify their argument.

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);
}
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It would be best if you made sure all the keys of %po existed in $process. How are you deciding priority? That could have a huge influence on how you actually do this.