While not strictly Perl-related, it was pointed out to me today that the Object-Oriented Reengineering Patterns book by Demeyer, Ducasse, and Nierstrasz is now available for free download. This book was originally published in 2003 and its copyright has now reverted to its authors.
I've long felt that there were insufficient good books available on reengineering old code. For Perl, Perl Medic: Transforming Legacy Code by Peter Scott is well worth a read. Can anyone recommend any other good references on dealing with legacy code?