Out of waffles. For such a loose specification: use Perl. IO bound stuff is unlikely to see much difference in performance unless you write the code badly in one or the other. Perl’s regexes are easier, deeper, and probably faster (depends on which perl 5.#.# you’ve got) if you write them well.

If you want a more serious answer you should ask the question with sample data, code, and more information about the actual (pre|post) processing problems involved.