#!/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