How to circumvent warning in batch mode

[Decebal <cldwesterhof@gmail.com>]

I have the following code:
emacs -batch -nw --eval='
  (let (
        (match-length)
        (reg-exp "^ +")
        (substitute-str "@")
        )
    (find-file "input")
    (goto-char (point-min))
    (while (re-search-forward "^ +" nil t)
      (setq match-length (- (point) (match-beginning 0)))
      (while (> match-length (length substitute-str))
        (setq substitute-str (concat substitute-str substitute-str)))
      (replace-match (substring substitute-str 0 match-length))
    )
    (write-file "outputEmacs")
  )
'
I have severall questions about it.
The input file is quite big and I get:
    File input is large (31MB), really open? (y or n)
Is there a way to circumvent this?
Is there a way to do this more efficient? This script needs about 20
seconds. When doing it with a Perl script, it takes about 6 seconds.
Instead of the '@' or chr$(64) I would like to use a nbsp or chr
$(160). But then the script needs almost 3 minutes. Also every space
is replaced by two characters chr$(194) + chr$(160).
What is going wrong here?