use warnings;
use strict;

@ARGV or die "Usage: cipher keyfile < source > dest\n";

open K,shift or die "Can't open keyfile: $!\n";
undef $/; my $k = <K>; close K; # slurp
length $k or die "error reading key file\n";

while (read STDIN, my $t, length $k)
{
  my @k = unpack "C*", $k; 
  print pack "C*", map { $_^shift @k } unpack "C*", $t;
}  

##</code><code>##

# make a key file, ok for demo purposes
$ perl -e 'print chr int rand 256 for 0..10000' > key.file

# make a test source file
$ ls -al /dev > plain.txt

# encipher
$ perl cipher.pl key.file < plain.txt > cipher.txt

# decipher
$ perl cipher.pl key.file < cipher.txt > deciphered.txt

# verify
$ diff -s plain.txt deciphered.txt 
Files plain.txt and deciphered.txt are identical