Most of that comes from reading "Working Effectively With Legacy Code".

Since you're already reading Working Effectively with Legacy Code a couple more PM nodes you might find interesting (this classic book is mentioned in both threads):

• Strategies for maintenance of horrible code? (discusses refactoring vs rewriting of very large code bases)
• Nobody Expects the Agile Imposition (Part VI): Architecture (many examples of legacy code, including perl 5 internals and even the Perl Monks code base)