Like author Peter Scott's previous book (co-written with Ed Wright), Perl Debugged, Perl Medic is rather inaccurately titled. Both books are collections of advice and tips on best practices and idioms in Perl, like Effective Perl Programming and Perl Best Practices. You can think of Perl Medic as a sequel to Perl Debugged. You definitely don't need to have read the former, but if you have, you should have a good idea about the format and contents you can expect here.
There is more advice on code style, low-level stuff like brace indentation and choosing decent variable names. Additionally, tracing, profiling, benchmarking and logging are again introduced. Some of this is admittedly rather reminiscent of what appeared in Perl Debugged. Happily, also present is the clear writing, which is informal without ever talking down to the reader.
Perl Medic, however, has a more agile flavour than Perl Debugged. Again, there is a chapter on unit testing, but it's much more fleshed out in this book, featuring far more modules, and providing an entire Test Driven Development example of a bounds-checked array.
Also, some refactorings are introduced, such as inlining temporary variables and extracting subroutines. The final chapter of the book is a refactoring example on some ancient Perl 4 cgi code, that pulls together most of the themes of the book.
There's also an in-depth discussion of what the strict and warnings pragmas entail, beyond that provided by most books. Also worthy of note is the chapter on 'cargo cult' Perl, where the author's pet peeves about non-idiomatic Perl code are explored, from practices which are merely pointless to the downright wrong.
Much of this material is general good practice. However, there are some chapters that help to justify the 'legacy code' approach. Chapter 7 provides a potted history of the different versions of Perl from Perl 4 onwards, and how to spot which version a program was targetted at. There is some mention of legacy code in the unit testing chapter, but not much.
In general, I think a lot of the suggestions will be obvious to anyone with experience with Perl, or experience with another modern programming language: use new modules from CPAN where you can, use lexically scoped (my) variables and localise global access and, of course, turn on strict and warnings. Overall, I wasn't much convinced with the 'legacy code' aspect of the book and the strained medical metaphor renders many of the chapter names nonsensical.
Another slight problem I had is that, like Perl Debugged, there can be a lack of flow between (and sometimes within) chapters. Chapter 10, nominally about maintainability, starts with assertions and Eiffel-style contract programming, and then goes onto logging, tieing, overloading, POD and version control integration. It's hard to see this as a cohesive whole.
If you're looking for a book on dealing with legacy code, Michael Feathers' Working Effectively with Legacy Code provides more bang for your buck -- although it's not Perl-specific (in fact, I'm pretty sure there's no Perl code in it at all). However, if you're a budding agilist looking for Perl-based examples of refactoring and TDD, then this is a good choice. Less experienced Perl programmers should definitely consider this, especially alongside Perl Debugged, as it provides both a good grounding in best practices, and more advanced development skills such as profiling and code coverage.