Add 'perl-non-scalar-variable' face to perl-mode

* etc/NEWS: Document the new face (bug#45840).
* lisp/progmodes/perl-mode.el (perl-non-scalar-variable): Define new
face.
(perl-font-lock-keywords-2): Apply 'perl-non-scalar-variable' face.

etc/NEWS

@@ -352,6 +352,12 @@ When emacsclient connects, Emacs will (by default) output a message
 about how to exit the client frame.  If 'server-client-instructions'
 is set to nil, this message is inhibited.
 
+** Perl mode
+
+---
+*** New face 'perl-non-scalar-variable'.
+This is used to fontify non-scalar variables.
+
 ** Python mode
 
 *** 'python-shell-interpreter' now defaults to python3 on systems with python3.

lisp/progmodes/perl-mode.el

@@ -95,6 +95,12 @@
   :prefix "perl-"
   :group 'languages)
 
+(defface perl-non-scalar-variable
+  '((t :inherit font-lock-variable-name-face :underline t))
+  "Face used for non-scalar variables."
+  :version "28.1"
+  :group 'perl)
+
 (defvar perl-mode-abbrev-table nil
   "Abbrev table in use in perl-mode buffers.")
 (define-abbrev-table 'perl-mode-abbrev-table ())
@@ -187,11 +193,12 @@
      ;;
      ;; Fontify function, variable and file name references.
      ("&\\(\\sw+\\(::\\sw+\\)*\\)" 1 font-lock-function-name-face)
-     ;; Additionally underline non-scalar variables.  Maybe this is a bad idea.
+     ;; Additionally fontify non-scalar variables.  `perl-non-scalar-variable'
+     ;; will underline them by default.
      ;;'("[$@%*][#{]?\\(\\sw+\\)" 1 font-lock-variable-name-face)
      ("[$*]{?\\(\\sw+\\(::\\sw+\\)*\\)" 1 font-lock-variable-name-face)
      ("\\([@%]\\|\\$#\\)\\(\\sw+\\(::\\sw+\\)*\\)"
-      (2 (cons font-lock-variable-name-face '(underline))))
+      (2 'perl-non-scalar-variable))
      ("<\\(\\sw+\\)>" 1 font-lock-constant-face)
      ;;
      ;; Fontify keywords with/and labels as we do in `c++-font-lock-keywords'.