Chapter 30. The Standard Perl Library
The standard Perl distribution contains much more than just the
perl executable that executes your scripts.
It also includes hundreds of modules filled with reusable code. Because the
standard modules are available everywhere, if you use one of them in
your program, you can run your program anywhere Perl is installed, without
any extra installation steps.
30.1. Library Science
Before we enumerate these modules in the following chapters, let's
review a bit of the terminology we've been splattering about.
A namespace is a place to keep names so they won't be confused with
names in other namespaces. This leaves you with the simpler problem of
not confusing the namespaces themselves. There are two ways to avoid
confusing namespaces with each other: give them unique names, or give
them unique locations. Perl lets you do both: named namespaces are
called packages and unnamed namespaces are called lexical scopes. Since
lexical scopes can be no larger than a file, and since the
standard modules are file-sized (at minimum), it follows that all
module interfaces must make use of named namespaces (packages) if
they're to be used by anyone outside the file.
A package is Perl's standard mechanism for declaring a named
namespace. It's a simple mechanism for grouping together related
functions and variables. Just as two directories can both contain a
(different) file named fred, two different parts of a Perl program
can each have its own $fred variable or &fred function. Even
though these variables or functions seem to have the same name as one
another, those names reside in distinct namespaces managed by the
package declaration. Package names are used to identify both
modules and classes, as described in Chapter 11, "Modules", and in
Chapter 12, "Objects".
The term library is unfortunately rather overloaded in Perl
culture. These days we normally use the term to mean the entire set of
Perl modules installed on your system.
Historically, a Perl library was also a single file containing a
collection of subroutines sharing some common purpose. Such a file
often has the file extension .pl, short for "perl library". We
still use that extension for random bits of Perl code that you pull in
with do FILE or with require. Although it's not a
full-fledged module, a library file typically declares itself to be in
a distinct package so related variables and subroutines can be kept
together and don't accidentally interfere with other
variables in your program. There is no mandatory extension; others
besides .pl sometimes occur as explained later in this
chapter. These simple,
unstructured library files have been largely superseded by the module.
A Perl module is a library file that conforms to
conventions that allow one or more files implementing that module to be
brought in with a single use declaration at compile
filenames must always end in .pm, because the
use declaration assumes
it. The use declaration will also translate the
package separator :: to whatever
your directory separator is, so that the directory structure in your
Perl library can match your package structure. Chapter 11, "Modules"
describes how to create your own Perl modules.
A class is just a module that implements methods
associated with the module's package name. If you're interested in
object-oriented modules, see Chapter 12, "Objects".
A pragma is just a special module that twiddles
Perl's internal knobs. See Chapter 31, "Pragmatic Modules".
An extension is a Perl module that, in addition to
loading a .pm file, also loads a shared library
implementing the module's semantics in C or C++.
A Perl program is code designed to be run as an independent entity;
also known as a script when you don't want anyone to expect much
from it, an application when it's big and complicated, an
executable when its caller doesn't care what language it was written
in, or an enterprise solution when it costs a fortune. Perl
programs might exist as source code, bytecode, or native machine code. If
it's something you might run from the command line, we'll call it
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