We recommend not to turn off indentation.

Really, don’t do it.

If you want to see what it’d be, you will have to turn off a lot of “electric-foo” commands (such as automatic newlines and indentation after {, ;, etc.

Here’s how to turn it off after TAB. First, we define a function to turn it all off, then add this function to all the modes where you want it turned off.

(defun my-turn-indentation-off ()
  (local-set-key (kbd "<tab>") 'tab-to-tab-stop))

(dolist (hook '(perl-mode-hook
  (add-hook hook 'my-turn-indentation-off))

Now all you have to do is customize either one of these two variables: ‘tab-stop-list’ or ‘tab-width’.

If you just want to indent to the previous line, always, replace the definition of ‘my-turn-indentation-off’ with the following:

(defun my-turn-indentation-off ()
  (local-set-key (kbd "<tab>") 'indent-relative))

As said before, your time is better spent customizing the indentation engines.