(defface cperl-my-trailing-spaces-face
      '((((class color))
         ;; define your tastes (background, foreground)
         (
          :background "grey98" 
;         :underline "grey"
          )
         ))
      "My face for trailing spaces in cperl mode"
      :group 'cperl-mode) ; Here is the most important part. It says to add
                          ; this face to the customizable group cperl-mode
    ;; now you can "bind" this face to cperl-invalid-face
    (set-default 'cperl-invalid-face 'cperl-my-trailing-spaces-face)