Perl can handle complex data structures. This is useful in many programming situations; it's also necessary to learn in order to read a lot of existing Perl code that might come your way.

For example, in this book, you've parsed a lot of data. To do so, you developed groups of subroutines, each fairly short, and each parsing different levels of the structure of the data. By using complex data structures, you can store your parse in a form that reflects the structure of the data. This, combined with object-oriented methods for accessing the parsed data, is a useful way to accomplish a parse.

Complex data structures depend on references, which I've touched on in discussions of call by reference and of File::Find.