(defun prefix-region (prefix)
  "Insert PREFIX at the beginning of each line between mark and point."
  (interactive "sPrefix string: ")
  (let ((end (region-end)))
    (save-excursion
      (goto-char (region-beginning))
      (beginning-of-line)
      (save-restriction
        (narrow-to-region (point) end)
        (while (not (eobp))
          (insert prefix)
          (forward-line 1))))))