Higher-Order Perl is about functional programming techniques in Perl. It's about how to write functions that can modify and manufacture other functions. Why would you want to do that? Because that way your code is more flexible and more reusable. Instead of writing ten similar functions, you write a general pattern or framework that can generate the functions you want; then you generate just the functions you need according to the pattern. The program doesn't need to know in advance which functions are necessary; it can generate them as needed. Instead of writing the complete program yourself, you get the computer to write it for you.
This absolutely great Perl book is now available for free download in PDF format by virtue of special permission from the publisher.