http://rzlab.ucr.edu/debian/public_emacs

...
set-key "\C-xp" 'gnuserv-edit)

;; Use c-mode for perl .xs files
(add-to-list 'auto-mode-alist '("\\.xs\\'" . c-mode))
(add-to-list 'auto-mode-alist '("\\.\\([pP][Llm]\\|al\\)\\'" . cperl-m
+ode))
(add-to-list 'interpreter-mode-alist '("perl" . cperl-mode))
(add-to-list 'interpreter-mode-alist '("perl5" . cperl-mode))
(add-to-list 'interpreter-mode-alist '("miniperl" . cperl-mode))

(setq cperl-hairy t
      cperl-indent-level 4
;;      cperl-auto-newline t
;;      cperl-auto-newline-after-colon t
      cperl-continued-statement-offset 4
      cperl-brace-offset -4
      cperl-label-offset -4
      cperl-highlight-variables-indiscriminately t
      cperl-electric-lbrace-space nil
      )
(add-hook 'cperl-mode-hook (lambda () (cperl-set-style "K&R")))

(setq c-indent-level 8)
(setq c-brace-imaginary-offset 0)
(setq c-brace-offset -8)
(setq c-argdecl-indent 8)
(setq c-label-offset -8)
...
[download]

As far as I can tell, the only thing relevant to XS there is

(add-to-list 'auto-mode-alist '("\\.xs\\'" . c-mode))
[download]

which is to say they set files ending in ".xs" to C mode. But XS files aren't C code. A snippet for example:

# -----------------------------------------

MODULE = Mozilla::DOM    PACKAGE = Mozilla::DOM::NSEvent    PREFIX = m
+oz_dom_

PRBool
moz_dom_DispatchEvent (target, event)
        nsIDOMEventTarget *target;
        nsIDOMEvent *event;
    PREINIT:
        PRBool rv;
    CODE:
        target->DispatchEvent(event, &rv);
        RETVAL = rv;
    OUTPUT:
        RETVAL
[download]

Though at a glance it looks like the syntax highlighting is okay in C mode. It would be neat if there was an XS mode, similar to mmm-mode in that it could use different modes in different regions of the page (e.g. in mmm-mode, the HTML is done differently than the %perl sections; the XS mode could distinguish the C code at the top from the XS code at the bottom).