FABER EST SUAE QUISQUE FORTUNAE APPIUS CLAUDIUS CAECUS DICTUM ARCANUM EST NEUTRON
I really enjoyed this book. Have you cracked some of the harder ones?
You could shorten your code quite a bit like this, not that it really matters
 
tachyon

use strict;

my @code=<DATA>;
my $code = join'', @code;
my @alphabet = ("A".."Z");
my (%crypt);

for my $try(0..26){
    print "\n---- Shift of $try :\n";
    for(A..Z){
        my $index= ($try++)%26;
        $crypt{$_} = $alphabet[$index];
    }
    for my $letter(split //,$code){
        ($letter =~/\s+|\n/)? print " " :print $crypt{$letter};
    }
}
print "\n";

__DATA__
MHILY  LZA  ZBHL  XBPZXBL  MVYABUHL  HWWPBZ  JSHBKPBZ  JHLJBZ
  KPJABT  HYJHUBT  LZA  ULBAYVU
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