So I had a glimpse into it.

Unfortunately the regex is not best quality, it allows preceding white-space to the POD commands.

Please note that cperl-mode already supports outline-mode, I didn't install outshine yet, but as a first step you should try to see if works out of the box without your modifications.

This should be your first step!

If outshine works and you don't like the levels:

see (defvar cperl-imenu--function-name-regexp-perl ... for the regex

and

(defun cperl-outline-level () for the prioritization of levels.

and adjust them to your needs (they already match everything you wanted except =item).

outline-level is just a function which maps any regex-group (number) to a level and the regex group numbers are very well documented in cperl.el

HTH!

Excerpt from cperl.el

;;; Details of groups in this are used in `cperl-imenu--create-perl-in
+dex'
;;;  and `cperl-outline-level'.
;;;; Was: 2=sub|package; now 2=package-group, 5=package-name 8=sub-nam
+e (+3)
(defvar cperl-imenu--function-name-regexp-perl
  (concat
   "^\\("                ; 1 = all
       "\\([ \t]*package"        ; 2 = package-group
          "\\("                ; 3 = package-name-group
        cperl-white-and-comment-rex ; 4 = pre-package-name
           "\\([a-zA-Z_0-9:']+\\)\\)?\\)" ; 5 = package-name
       "\\|"
          "[ \t]*sub"
      (cperl-after-sub-regexp 'named nil) ; 8=name 11=proto 14=attr-st
+art
      cperl-maybe-white-and-comment-rex    ; 15=pre-block
   "\\|"
     "=head\\([1-4]\\)[ \t]+"        ; 16=level
     "\\([^\n]+\\)$"            ; 17=text
   "\\)"))

(defvar cperl-outline-regexp
  (concat cperl-imenu--function-name-regexp-perl "\\|" "\\`"))

;; ...

;; Suggested by Mark A. Hershberger
(defun cperl-outline-level ()
  (looking-at outline-regexp)
  (cond ((not (match-beginning 1)) 0)    ; beginning-of-file
;;;; 2=package-group, 5=package-name 8=sub-name 16=head-level
    ((match-beginning 2) 0)        ; package
    ((match-beginning 8) 1)        ; sub
    ((match-beginning 16)
     (- (char-after (match-beginning 16)) ?0)) ; headN ==> N
    (t 5)))                ; should not happen
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