(defun my-perltidy-replace (arg)
  "Replace the contents of the current buffer with the output of
perltidy, and make a backup of the current buffer.

Before and after modifications to contents of buffers are being
made, all buffers that need to be auto-saved are being
auto-saved.

The major-mode of the buffer containing the backup is set to
cperl-mode."
  (interactive "P")
  (do-auto-save)
  (let ((tidy_buffer (generate-new-buffer (generate-new-buffer-name (concat "TidyBackup-" (buffer-name))))))
    (with-current-buffer (buffer-name)
      ;; swapping the text can screw up linum mode with two buffers in
      ;; the same frame when automatically enabled by cperl-mode
      (linum-mode -1)
      (shell-command-on-region (point-min) (point-max) "perltidy --standard-output" tidy_buffer)
      (buffer-swap-text tidy_buffer))
    (with-current-buffer tidy_buffer
      (auto-save-mode nil)
      (do-auto-save)
      (cperl-mode))
    (linum-mode 1)
    (message "buffer contents replaced with output of perltidy; backup is in %s" tidy_buffer)))

(eval-after-load 'cperl-mode
  '(define-key cperl-mode-map (kbd "C-x t i") 'my-perltidy-replace))