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I am using cperl-mode to edit Perl scripts. According to the Emacs wiki ( link ) one could use defalias to use cperl-mode instead of perl-mode. Here is an example:

$ emacs -Q -l init.el test.pl &

where init.el is:

(defalias 'perl-mode 'cperl-mode)
(defun my-test-hook ()
  (message-box (symbol-name major-mode)))
(add-hook 'cperl-mode-hook #'my-test-hook)

This gives two message boxes, indicating that the hook my-test-hook is being run twice at startup.. (I also tested this with emacs-lisp-mode (without using defalias of course) and then the hook was run only a single time, as expected.. So that was what lead me to suspect defalias was the culprit).

Why is the hook being run twice here? (I am asking because I would like to have it run only a single time if possible)

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  • I really doubt the defalias plays any role here. cperl-mode-hook variable is just a variable (possibly autoloaded), adding an alias to a function cperl-mode shouldn't affect it at all. I'd try to debug the cperl-mode function instead--maybe it runs the hooks twice in some cases?
    – wvxvw
    Mar 19 '15 at 14:22
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It looks like a bug in cperl-mode. It is defined using define-derived-mode, a special form that defines a mode. When using it, it generates code to call the mode hook. However, the body of also contain an explicit call to the hooks, so it will be called twice.

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