#!/usr/bin/perl

use strict;
use warnings;
no warnings qw /syntax/;

tie my %h => 'main';

sub TIEHASH {bless []}  
sub FETCH   {chr (ord ('A') + int rand 26)}  

while (<DATA>) {
    s/(\w)/$h{$1}/g;
    print;
}

__DATA__
ABCDEF