#!/usr/bin/perl

use strict;

use warnings;

while(<DATA>){

	chomp;
	my $in = $_;
	my @pairs = split(/(.{2})/);
	
	$_ = reverse foreach @pairs;

	my $out = join('', @pairs);

	print "in: $in => out: $out\n";

	}

__DATA__
12345678
1234
12
1234
123456
87654321

##</code><code>##

in: 12345678 => out: 21436587
in: 1234 => out: 2143
in: 12 => out: 21
in: 1234 => out: 2143
in: 123456 => out: 214365
in: 87654321 => out: 78563412