Snippet for StackOverflow comment (http://stackoverflow.com/a/13632665/1225607)

gistfile1.el

(defun isend--perl (buf-name)
  "Prepend 'x ' to normal perl instructions.
Leave 'print' instructions untouched."
  (with-current-buffer buf-name
    (goto-char (point-min))
    (unless (looking-at "[[:space:]]*print")
      (insert "x ")))
  (insert-buffer-substring buf-name))

(defun isend-default-perl-setup ()
  (when (eq major-mode 'perl-mode)
    (set (make-local-variable 'isend-send-line-function) #'isend--perl)))

(add-hook 'isend-mode-hook #'isend-default-perl-setup)