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) From the menu, Perl->Tools->Tags->Create tags for current file
   (or any other entry which creates tags from the same submenu)

This fails with "cond: Buffer is read-only: #<buffer TAGS>".

I *guess* this is related to recent changes in etags.pl which need to be
accomodated for in cperl-mode.  The function works fine in Emacs 26.1
(as available in Debian Buster.

Attached is a one-line fix, but I am unsure whether this is the right
way to deal with it.

Note 1: As a workaround one could of course use cperl-etags
        (which is an interactive function), but this is also
        broken, albeit in a different way.  Already reported
        as (bug#42355).

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:
Quit [2 times]
Making completion list... [2 times]
Auto-saving...done
<left-fringe> <mouse-1> is undefined [2 times]
Mark set [2 times]
Auto-saving...
Beginning of buffer [4 times]
user-error: Beginning of history; no preceding item
user-error: End of history; no default available [3 times]
completing-read-default: Command attempted to use minibuffer while in
minibuffer

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: Help

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:
(pp 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 133165 14283)
 (symbols 48 10626 1)
 (strings 32 36309 2069)
 (string-bytes 1 1030321)
 (vectors 16 17217)
 (vector-slots 8 293029 19492)
 (floats 8 56 44)
 (intervals 56 1604 21)
 (buffers 992 21))