Mastering Perl is a 400-page book by Brian D. Foy that teaches advanced Perl programming concepts. It focuses on helping readers solve real-world problems like debugging, configuration, and security as a working programmer. The book explores topics like advanced regular expressions, avoiding common security issues, profiling code performance, and manipulating packages and modules. Mastering Perl aims to help readers bend the Perl language to their needs and think like experienced Perl programmers.