(defun count-prefix-spaces-in-lines (pathname)
  (with-open-file (stream pathname)
    (loop :for line := (read-line stream nil)
          :while line
          :collect (loop :for ch :across line
                         :while (find ch #(#\space #\tab))
                         :sum 1))))


(count-prefix-spaces-in-lines "/etc/passwd")
--> (0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0)