#!/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 }