I'd second this comment.

I took over a large and mostly monolithic application. At the time there were no test cases and so I went through and did two things at once:

• Wrote test cases against an imaginary modular-API.
• Broke the code into many small modules.

Over time 99% of the code has been turned into objects and modules and now I have a good place to add more tests.

Testing a large monolithic application is quite hard.

Although if you can install an instance upon a test environment (perhaps along with a test database with contents, etc) you may be able to use the WWW::Mechanize family of modules to fire requests against it and parse the results back out.