#!/usr/bin/perl

use strict;
use warnings;

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

my $cont = do {
    open my $fh, '<:raw', $fn or die "$fn: D'Oh! $!\n";
    local $/;
    <$fh>;
};

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

print $fh $cont ^ do { 
    "\0" x rand(-$LEN + length $cont) .
      join '', map { chr rand 0x100 } 1 .. $LEN;
};

__END__