How to reproduce:

1) Open a file with arbitrary Perl code, for example like that:
   ----
   package Tag::It;

   sub bark {
       say 'woof!';
   }
   ----
2) M-x cperl-mode
3) M-x cperl-etags

This fails with (wrong-type-argument stringp cperl-sub-regexp).
The error came with incorporating Jonathan Rockway's work.

A patch is attached.

Note 1: As a workaround one could of course use cperl-write-tags
        (which is available from the menu), but this is also
        broken, albeit in a different way, and only in master.

Note 2: No need to remind me of copyright paperwork:  I've sent
        the form to assign@gnu.org on July 02, and pinged them
        again on July 13.

In GNU Emacs 28.0.50 (build 1, x86_64-pc-linux-gnu, X toolkit, Xaw
scroll bars)
 of 2020-07-07 built on hajtower
Repository revision: 6b80ff3c465f87a31ccaaf41b2b521075f43632d
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12004000
System Description: Debian GNU/Linux 10 (buster)

Recent messages:
Source file ‘/home/haj/devel/emacs/emacs/lisp/gnus/message.el’ newer
than byte-compiled file; using older file
Source file ‘/home/haj/devel/emacs/emacs/lisp/gnus/gmm-utils.el’ newer
than byte-compiled file; using older file
Source file ‘/home/haj/devel/emacs/emacs/lisp/gnus/mml.el’ newer than
byte-compiled file; using older file
Source file ‘/home/haj/devel/emacs/emacs/lisp/gnus/mm-decode.el’ newer
than byte-compiled file; using older file
Source file ‘/home/haj/devel/emacs/emacs/lisp/gnus/gnus-util.el’ newer
than byte-compiled file; using older file
Source file ‘/home/haj/devel/emacs/emacs/lisp/mail/rmail.el’ newer than
byte-compiled file; using older file
Source file ‘/home/haj/devel/emacs/emacs/lisp/epa.el’ newer than
byte-compiled file; using older file
Source file ‘/home/haj/devel/emacs/emacs/lisp/password-cache.el’ newer
than byte-compiled file; using older file
Source file ‘/home/haj/devel/emacs/emacs/lisp/dired.el’ newer than
byte-compiled file; using older file
Source file ‘/home/haj/devel/emacs/emacs/lisp/net/puny.el’ newer than
byte-compiled file; using older file

Configured features:
XPM JPEG TIFF GIF PNG SOUND NOTIFY INOTIFY GNUTLS LIBXML2 ZLIB
TOOLKIT_SCROLL_BARS LUCID X11 XDBE XIM MODULES THREADS PDUMPER GMP

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Info

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-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
  buffer-read-only: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny dired dired-loaddefs
rfc822 mml mml-sec password-cache epa epg epg-config gnus-util rmail
rmail-loaddefs text-property-search mm-decode mm-bodies mm-encode
mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail rfc2047
rfc2045 ietf-drums mm-util mail-prsvr mail-utils apropos mule-util
misearch multi-isearch bug-reference cl-extra seq cl-print cus-edit
wid-edit cus-start cus-load thingatpt help-fns radix-tree help-mode
perl-mode time-date subr-x vc-git diff-mode easy-mmode cperl-mode
derived info man ansi-color etags fileloop generator xref cl-seq project
eieio byte-opt bytecomp byte-compile cconv eieio-core cl-macs gv
eieio-loaddefs ring easymenu imenu cl-loaddefs cl-lib tooltip eldoc
electric uniquify ediff-hook vc-hooks lisp-float-type mwheel term/x-win
x-win term/common-win x-dnd tool-bar dnd fontset image regexp-opt fringe
tabulated-list replace newcomment text-mode elisp-mode lisp-mode
prog-mode register page tab-bar menu-bar rfn-eshadow isearch timer
select scroll-bar mouse jit-lock font-lock syntax facemenu font-core
term/tty-colors frame minibuffer 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 composite charscript charprop case-table epa-hook
jka-cmpr-hook help simple abbrev obarray 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 threads inotify dynamic-setting x-toolkit x multi-tty
make-network-process emacs)

Memory information:
((conses 16 132059 6725)
 (symbols 48 10621 1)
 (strings 32 36250 1911)
 (string-bytes 1 1024176)
 (vectors 16 16428)
 (vector-slots 8 286664 13086)
 (floats 8 51 23)
 (intervals 56 1436 0)
 (buffers 992 20))