#!/usr/bin/perl
use strict;

my @set = ('A', 'B', 'C', 'D', 'E', 'F');
my $length = 12;

my $result;

my $last_char;
while(length($result) < $length) {
     my $char = $set[int(rand()*(scalar(@set)-1))];
     next if ( $last_char eq $char);
     $result .= $char;
     $last_char = $char;
}

print $result;