(defun insert-str-with-line-context (regexp inserting-str default-char)
    (let ((ptr (point)))
      (beginning-of-line)
      (let ((str (buffer-substring (point) ptr)))
        (goto-char ptr)
        (if (string-match regexp str)
            (insert inserting-str)
          (insert default-char)))))

;; typing "4" inserts "$", not "4" if you wrote "my " or "my ("
(define-key cperl-mode-map (kbd "4")
  (lambda () (interactive)
    (insert-str-with-line-context "[ \t]*my[ ]+(?$" "$" "4")))
(define-key cperl-mode-map (kbd "8")
  (lambda () (interactive)
    (insert-str-with-line-context "[ \t]*my[ ]+$" "(" "8")))
;; typing ":" insert "::" after writing "package ***"
(define-key cperl-mode-map (kbd ":")
  (lambda () (interactive)
    (insert-str-with-line-context "^package" "::" ":")))