How's this?

#!/usr/bin/perl

use strict;
use warnings;

my $input = 'a';
my %po =    (
                PR1    =>    2,
                PR2    =>    4,
                PR3    =>    1,
                PR4    =>    3,
            );

foreach my $key (sort { $po{$a} <=> $po{$b} } keys %po) {

    $key =~ s/^PR/process/;
    $input = eval "$key(\$input)" or die $@;

    print $input . $/; # debugging output

}

sub process1 { lc(shift) . 1 }
sub process2 { lc(shift) . 2 }
sub process3 { uc(shift) . 3 }
sub process4 { uc(shift) . 4 }
[download]

Output:

A3
a31
A314
a3142
[download]

Update: eval can be dangerous! Use this solution only if you are sure that the value of $input is safe! Taint mode highly recommended.