#!/usr/bin/perl -wl
use strict;
my @ari;
my $str = "17341234173412341734123417341234";

$ari[2] = $str;
substr($ari[2], $_*4, 8) =~ s[(.)(?=...\1)|(?<=(.)...)\2]{
    defined $1 ? "A" : "B"
}eg for 0 .. length($ari[2])/4;
print $ari[2];
__END__
A7AAB2BBA7AAB2BBA7AAB2BBA7AAB2BB