use warnings;
use strict;

my $cipher = shift;
my $key    = shift;

open( my $cipher_fh, $cipher ) or die $!;
open( my $key_fh,    $key )    or die $!;

my $buffer_length = 4_096;

while ( sysread( $cipher_fh, my $cipher_text, $buffer_length, )
     && sysread( $key_fh,    my $key_text,    $buffer_length, ) ) {

         print $cipher_text ^ $key_text;
}