I haven't used pcase extensively, but based on the warning, the below doesn't work as intended in the compiled elc:
((or "C" "L")
(let ((insert-fun (if (equal key "C") #'insert
(lambda (s) (org-insert-link 0 s))))
(first-value (car org-capture--clipboards)))
(pcase (length org-capture--clipboards)
(`nil nil)
(`(,value) (funcall insert-fun value))
(`(,first-value . ,_)
(funcall insert-fun
(read-string "Clipboard/kill value: "
first-value ; Based on the warning, does it mean that the value of first-value is not picked from the value calculated in the let form above?
'org-capture--clipboards
first-value))) ; And the same here
(_ (error "Invalid `org-capture--clipboards' value: %S"
org-capture--clipboards)))))