Basically you want something along the lines of:

#
# Warning: untried code
#
my $message = "Blah!";
my (%translation);
my $alphabet = join('', (a..z));
# Cheezy, but does 2 passes, performing a uc() in between
foreach (0..1) {
  my $startchar = ord(substr($alphabet, 0, 1));
  foreach my $c (0..25) {
    $translation{chr($startchar + $_)} = 
      substr($alphabet, $_ - 13, 1);
  }
  $alphabet = uc($alphabet);
}
# %translation now contains the value for each character 
#  in ('a'..'z', 'A'..'Z')
print("Old message: ", $message, "\n");
my @letters = split('', $message); 
my (@newmessage);
while (@letters) {
  my $c = shift(@letters);
  $c = $translation{$c} 
    if (defined($translation{$c}));
  push(@newmessage, $c);
}
$message = join('', @newmessage);
print("New message: ", $message, "\n");
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Others may know a better way to implement it, and I look forward to seeing their postings. Hope it at least helps.