Having read and very much enjoyed this book a few years ago I took a stab at this particular cipher. Your letter crunching was a bit cleaner than mine so I adapted my old script - I split the key into an array to avoid having to create a second string the same size as the plaintext which may be quite large.

#!/usr/bin/perl -lw
use constant ENCRYPT => 1;
use constant DECRYPT => -1;
use strict;

my $txt = 'A Perlmonk\'s work is never done!';
my $key = 'larryk';

$txt =~ s/[^a-z]//gi;

print 'before   : ',$txt;
print 'encrypted: ',encrypt( $txt, $key );
print 'after    : ',decrypt( encrypt( $txt, $key ), $key );

sub encrypt {
    my @txt = split '', shift;
    my @key = split '', shift;
    join '', map uc letter( ENCRYPT, $txt[$_], $key[$_%@key] ), 0..$#t
+xt;
}

sub decrypt {
    my @txt = split '', shift;
    my @key = split '', shift;
    join '', map lc letter( DECRYPT, $txt[$_], $key[$_%@key] ), 0..$#t
+xt;
}

sub letter {
    my ($direction,$cipherchar,$keychar) = @_;
    lc chr( 97 + (              (ord(lc $cipherchar) - 97)
                   + $direction*(ord(lc $keychar   ) - 97) ) % 26 );
}
[download]

   larryk                                          
perl -le "s,,reverse killer,e,y,rifle,lycra,,print"

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