#!/usr/bin/perl
use strict;
use warnings;
use Fcntl ':seek';

die "Usage: $0 file\n" unless @ARGV == 1;
my $fn  = shift;
my $LEN = 3;

open my $fh, '+<:raw', $fn or die "$fn: D'Oh! $!\n";

my $offset = rand( -$LEN + -s $fh );

seek $fh, $offset, SEEK_SET or die "$fn: D'Oh! $!\n";
read $fh, my $cont, $LEN;
seek $fh, $offset, SEEK_SET or die "$fn: D'Oh! $!\n";
print $fh $cont ^ join '', map chr( rand 0x100 ), 1 .. $LEN;

__END__