I'm currently working on a big, and I mean /BIG/ codebase, like 200K LOC with about 10 years of history behind it.
In this post I'll briefly describe how I'm refactoring code using a little tool called Geest (github), which I completely stole from Ruby's kage.
tl;dr: With Geest you can check differences between new / old code transparently. It's really handy. Please let me know what you think, or file issues if you find any.
Japan Perl Association director; Livedoor, Inc; Tokyo, Japan