home | O'Reilly's CD bookshelfs | FreeBSD | Linux | Cisco | Cisco Exam  

Learning Perl

Learning PerlSearch this book
Previous: A.18 Chapter 19, CGI Programming Appendix B Next: B.2 Standard Modules

B. Libraries and Modules

For simple programs you can easily write your own Perl routines and subroutines. As the tasks to which you apply Perl become more difficult, however, sometimes you'll find yourself thinking, "someone must have done this already." You are probably more right than you imagine.

For most common tasks, other people have already written the code. Moreover, they've placed it either in the standard Perl distribution or in the freely downloadable CPAN archive. To use this existing code (and save yourself some time), you'll have to understand how to make use of a Perl library. This was briefly discussed in Chapter 19, CGI Programming .

One advantage in using modules from the standard distribution is that you can then share your program with others without their having to take any special steps. This is because the same standard library is available to Perl programs almost everywhere.

You'll save yourself time in the long run if you get to know the standard library. There's no point in reinventing the wheel. You should be aware, however, that the library contains a wide range of material. While some modules may be extremely helpful, others may be completely irrelevant to your needs. For example, some are useful only if you are creating extensions to Perl.

To read the documentation for a standard module, use the man or perldoc program (if you have them), or perhaps your web browser on HTML versions of the documentation. If all else fails, just look in the module itself: the documentation is contained within each module in pod format. To locate the module on your system, try executing this Perl program from the command line:

# for (most) Unix-like shells
perl -e 'print "@INC\n"'
# for (some) other command interpreters
perl -e "print join(' ',@INC),\n"

You should find the module in one of the directories listed by this command.

B.1 Library Terminology

Before we list all the standard modules, let's untangle some terminology:


A package is a simple namespace management device, allowing two different parts of a Perl program to have a (different) variable named $fred . These namespaces are managed with the package declaration, described in Chapter 5 of Programming Perl .


A library is a set of subroutines for a particular purpose. Often the library declares itself a separate package so that related variables and subroutines can be kept together, and so that they won't interfere with other variables in your program. Generally, an old-style library was placed in a separate file, often with a name ending in ".pl". The library routines were then pulled into the main program via the require function. More recently this older approach has been replaced by the use of modules (see next paragraph), and the term library often refers to the entire system of modules that come with Perl.


A module is a library that conforms to specific conventions, allowing the library routines to be brought into your program with the use directive at compile-time. Module filenames end in ".pm", because the use directive insists on that. Chapter 5 of Programming Perl describes Perl modules in greater detail.


A pragma is a module that affects the compilation phase of your program as well as the execution phase. Think of it as containing hints to the compiler. Unlike other modules, pragmas often (but not always) limit the scope of their effects to the innermost enclosing block of your program (that is, the block enclosing the pragma invocation). The names of pragmas are by convention all lowercase.