It seems to me that an Emacs shortcut to avoid the mistake you describe would be significantly complicated by the fact that the LHS of the assignment is quite variable, but maybe this will be enough:

(add-hook 'cperl-mode-hook (lambda()
                             (local-set-key "\C-cm" 'perl-insert-oo-me
+thod)))

(defun perl-insert-oo-method (methodname)
  "Insert Perl OO method"
  (interactive "MEnter method's name: ")
  (insert
    (format "sub %s {\nmy ( $self ) = @_;" methodname))
  (backward-char 8)
  (cperl-indent-command)
)
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Tweak to taste. It assumes that you are using cperl-mode (and if you aren't I highly recommend it). Also, you can probably modify this example to handle other common coding shortcuts.

Update:For those less familiar with extending Emacs:

1. Cut and paste the code above to your .emacs file or equivalent (I believe that for xemacs that would be the file .xemacs/custom.el, but I'm not sufficiently familiar with xemacs to know for sure).
2. Make this inserted text the region, and execute the command eval-region (i.e. M-x eval-region).
3. Switch to a new buffer and execute he command cperl-mode (i.e. M-x cperl-mode).
4. Now, in cperl mode, the key sequence C-c m will bring up a prompt for the method name:

   Enter method name: foo
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   Once the method name is entered (say foo), the following will be inserted at point:

   sub foo {
     my ( $self ) = @_;
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   and point will be placed right after $self.