bug#21170: 25.0.50; xref doesn't work with cperl tags

[Alexis Layton <alex@take-the-blame.permabit.com>]

M-. (xref-find-definitions) does not seem to find tags in Perl files
the way find-tag used to. Our TAGS file has both C and Perl tags in
it. The C tags still work. The Perl tags claim 'no definitions found'.
(It does prompt for the location of the TAGS file as it used to.)
Note that we use cperl-mode if that matters.


In GNU Emacs 25.0.50.3 (x86_64-unknown-linux-gnu, GTK+ Version 3.10.8)
 of 2015-07-21 on take-the-blame.permabit.com
Repository revision: 492ea2410c6aab25a1d02cb576bb1df536c3f57c
Windowing system distributor `The X.Org Foundation', version 11.0.11501000
System Description:	Ubuntu 14.04.2 LTS

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GCONF GSETTINGS
NOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11

Important settings:
  value of $LANG: C.UTF-8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

Major mode: CPerl

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Recent messages:
Loading grep...done
Loading /permabit/user/alex/.emacs.d/lisp/custom-23.el (source)...done
Loading /permabit/user/alex/.emacs.d/lisp/alex-custom.el (source)...
Loading alex-perforce...done
Loading /permabit/user/alex/.emacs.d/lisp/alex-custom.el (source)...done
Loading vc-p4...done
For information about GNU Emacs and the GNU system, type C-h C-a.
Starting a new list of tags tables
user-error: No definitions found for: assertEqualNumeric
Making completion list...

Load-path shadows:
~/.emacs.d/lisp/diff-mode hides /usr/local/share/emacs/25.0.50/lisp/vc/diff-mode

Features:
(shadow sort gnus-util mail-extr emacsbug message format-spec rfc822 mml
mml-sec mm-decode mm-bodies mm-encode mail-parse rfc2231 mailabbrev
gmm-utils mailheader sendmail rfc2047 rfc2045 ietf-drums mm-util
help-fns mail-prsvr mail-utils etags thingatpt xref cl-seq project eieio
byte-opt bytecomp byte-compile cl-extra help-mode seq cconv eieio-core
cl-macs gv vc cl-loaddefs pcase cl-lib vc-dispatcher cperl-mode easymenu
vc-p4 p4-lowlevel p4 dired grep compile comint ansi-color ring cus-start
cus-load finder-inf go-mode-autoloads package epg-config time-date
mule-util tooltip eldoc electric uniquify ediff-hook vc-hooks
lisp-float-type mwheel x-win term/common-win x-dnd tool-bar dnd fontset
image regexp-opt fringe tabulated-list newcomment elisp-mode lisp-mode
prog-mode register page menu-bar rfn-eshadow timer select scroll-bar
mouse jit-lock font-lock syntax facemenu font-core frame cl-generic cham
georgian utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao
korean japanese eucjp-ms cp51932 hebrew greek romanian slovak czech
european ethiopic indian cyrillic chinese charscript case-table epa-hook
jka-cmpr-hook help simple abbrev minibuffer cl-preloaded nadvice
loaddefs button faces cus-face macroexp files text-properties overlay
sha1 md5 base64 format env code-pages mule custom widget
hashtable-print-readable backquote dbusbind gfilenotify dynamic-setting
system-font-setting font-render-setting move-toolbar gtk x-toolkit x
multi-tty make-network-process emacs)

Memory information:
((conses 16 167624 8176)
 (symbols 48 25769 0)
 (miscs 40 49 104)
 (strings 32 35204 6243)
 (string-bytes 1 1048948)
 (vectors 16 17795)
 (vector-slots 8 499295 7784)
 (floats 8 200 91)
 (intervals 56 1703 0)
 (buffers 976 14)
 (heap 1024 43767 1098))