re: Apprenticeship

(let ((e-mail-address "PGIUHOGT@VGIHKRR.TKZ"))
  (loop with new-string = (make-string (length e-mail-address))
    for count from 0 to (1- (length e-mail-address))
    for char-code = (char-code (aref e-mail-address count))
    for new-char-code =
      (if (and (> char-code 64)(< char-code 123))
        (+ (mod (+ 13 char-code) 52) 65)
        char-code)
    do (setf (aref new-string count) (code-char new-char-code))
    finally (return new-string)))

Gauche Scheme

(string-map
  (^c (let1 n (char->integer c)
        (integer->char
          (if (< 64 n 123) (+ (modulo (+ 13 n) 52) 65) n))))
  "PGIUHOGT@VGIHKRR.TKZ")