NAME

DESCRIPTION

Packages

(The old package delimiter was a single quote, but double colon is now the preferred delimiter, in part because it's more readable to humans, and in part because it's more readable to emacs macros. It also makes C++ programmers feel like they know what's going on.)

Packages may be nested inside other packages: $OUTER::INNER::var . This implies nothing about the order of name lookups, however. All symbols are either local to the current package, or must be fully qualified from the outer package name down. For instance, there is nowhere within package OUTER that $INNER::var refers to $OUTER::INNER::var . It would treat package INNER as a totally separate global package.

Only identifiers starting with letters (or underscore) are stored in a package's symbol table. All other symbols are kept in package main . In addition, the identifiers STDIN, STDOUT, STDERR, ARGV, ARGVOUT, ENV, INC and SIG are forced to be in package main , even when used for other purposes than their built-in one. Note also that, if you have a package called m , s or y , then you can't use the qualified form of an identifier because it will be interpreted instead as a pattern match, a substitution, or a translation.

(Variables beginning with underscore used to be forced into package main, but we decided it was more useful for package writers to be able to use leading underscore to indicate private variables and method names.)

Eval() ed strings are compiled in the package in which the eval() was compiled. (Assignments to $SIG {} , however, assume the signal handler specified is in the main package. Qualify the signal handler name if you wish to have a signal handler in a package.) For an example, examine perldb.pl in the Perl library. It initially switches to the DB package so that the debugger doesn't interfere with variables in the script you are trying to debug. At various points, however, it temporarily switches back to the main package to evaluate various expressions in the context of the main package (or wherever you came from). See the perldebug manpage .

Symbol Tables

The value in each entry of the associative array is what you are referring to when you use the *name notation. In fact, the following have the same effect, though the first is more efficient because it does the symbol table lookups at compile time:

You can use this to print out all the variables in a package, for instance. Here is dumpvar.pl from the Perl library:

Note that even though the subroutine is compiled in package dumpvar , the name of the subroutine is qualified so that its name is inserted into package main .

Assignment to a symbol table entry performs an aliasing operation, i.e.,

causes variables, subroutines and file handles accessible via the identifier richard to also be accessible via the symbol dick . If you only want to alias a particular variable or subroutine, you can assign a reference instead:

makes $richard and $dick the same variable, but leaves @richard and @dick as separate arrays. Tricky, eh?

Package Constructors and Destructors

A BEGIN subroutine is executed as soon as possible, that is, the moment it is completely defined, even before the rest of the containing file is parsed. You may have multiple BEGIN blocks within a file--they will execute in order of definition. Because a BEGIN block executes immediately, it can pull in definitions of subroutines and such from other files in time to be visible to the rest of the file.

An END subroutine is executed as late as possible, that is, when the interpreter is being exited, even if it is exiting as a result of a die() function. (But not if it's is being blown out of the water by a signal--you have to trap that yourself (if you can).) You may have multiple END blocks within a file--they will execute in reverse order of definition; that is: last in, first out (LIFO).

Note that when you use the -n and -p switches to Perl, BEGIN and END work just as they do in awk , as a degenerate case.

Perl Classes

Perl Modules

Perl modules are included by saying

or

This is exactly equivalent to

or

All Perl module files have the extension .pm . use assumes this so that you don't have to spell out " Module.pm " in quotes. This also helps to differentiate new modules from old .pl and .ph files. Module names are also capitalized unless they're functioning as pragmas, "Pragmas" are in effect compiler directives, and are sometimes called "pragmatic modules" (or even "pragmata" if you're a classicist).

Because the use statement implies a BEGIN block, the importation of semantics happens at the moment the use statement is compiled, before the rest of the file is compiled. This is how it is able to function as a pragma mechanism, and also how modules are able to declare subroutines that are then visible as list operators for the rest of the current file. This will not work if you use require instead of use . Therefore, if you're planning on the module altering your namespace, use use ; otherwise, use require . Otherwise you can get into this problem:

Perl packages may be nested inside other package names, so we can have package names containing :: . But if we used that package name directly as a filename it would makes for unwieldy or impossible filenames on some systems. Therefore, if a module's name is, say, Text::Soundex , then its definition is actually found in the library file Text/Soundex.pm .

Perl modules always have a .pm file, but there may also be dynamically linked executables or autoloaded subroutine definitions associated with the module. If so, these will be entirely transparent to the user of the module. It is the responsibility of the .pm file to load (or arrange to autoload) any additional functionality. The POSIX module happens to do both dynamic loading and autoloading, but the user can just say use POSIX to get it all.

For more information on writing extension modules, see the perlapi manpage and the perlguts manpage .

NOTE

The module and its user have a contract, part of which is common law, and part of which is "written". Part of the common law contract is that a module doesn't pollute any namespace it wasn't asked to. The written contract for the module (AKA documentation) may make other provisions. But then you know when you use RedefineTheWorld that you're redefining the world and willing to take the consequences.

THE PERL MODULE LIBRARY

Pragmatic Modules

which lasts until the end of that BLOCK.

The following programs are defined (and have their own documentation).

integer

Perl pragma to compute arithmetic in integer instead of double

less

Perl pragma to request less of something from the compiler

sigtrap

Perl pragma to enable stack backtrace on unexpected signals

strict

Perl pragma to restrict unsafe constructs

subs

Perl pragma to predeclare sub names

Standard Modules

Abbrev

create an abbreviation table from a list

AnyDBM_File

provide framework for multiple DBMs

AutoLoader

load functions only on demand

AutoSplit

split a package for autoloading

Basename

parse file name and path from a specification

Benchmark

benchmark running times of code

Carp

warn or die of errors (from perspective of caller)

CheckTree

run many filetest checks on a tree

Collate

compare 8-bit scalar data according to the current locale

Config

access Perl configuration option

Cwd

get pathname of current working directory

DynaLoader

Dynamically load C libraries into Perl code

English

use nice English (or awk ) names for ugly punctuation variables

Env

Perl module that imports environment variables

Exporter

module to control namespace manipulations

Fcntl

load the C Fcntl.h defines

FileHandle

supply object methods for filehandles

Find

traverse a file tree

Finddepth

traverse a directory structure depth-first

Getopt

basic and extended getopt(3) processing

MakeMaker

generate a Makefile for Perl extension

Open2

open a process for both reading and writing

Open3

open a process for reading, writing, and error handling

POSIX

Perl interface to IEEE 1003.1 namespace

Ping

check a host for upness

Socket

load the C socket.h defines

Extension Modules

The following are popular C extension modules, which while available at Perl 5.0 release time, do not come bundled (at least, not completely) due to their size, volatility, or simply lack of time for adequate testing and configuration across the multitude of platforms on which Perl was beta-tested. You are encouraged to look for them in archie(1L), the Perl FAQ or Meta-FAQ, the WWW page, and even with their authors before randomly posting asking for their present condition and disposition. There's no guarantee that the names or addresses below have not changed since printing, and in fact, they probably have!

Curses

Written by William Setzer< William_Setzer @ncsu .edu >, while not included with the standard distribution, this extension module ports to most systems. FTP from your nearest Perl archive site, or try

ftp://ftp.ncsu.edu/pub/math/wsetzer/cursperl5??.tar.gz

It is currently in alpha test, so the name and ftp location may change.

DBI

This is the portable database interface written by < Tim.Bunce @ig .co.uk >. This supersedes the many perl4 ports for database extensions. The official archive for DBperl extensions is ftp.demon.co.uk:/pub/perl/db . This archive contains copies of perl4 ports for Ingres, Oracle, Sybase, Informix, Unify, Postgres, and Interbase, as well as rdb and shql and other non-SQL systems.

DB_File

Fastest and most restriction-free of the DBM bindings, this extension module uses the popular Berkeley DB to tie() into your hashes. This has a standardly-distributed man page and dynamic loading extension module, but you'll have to fetch the Berkeley code yourself. See DB_File for where.

Sx

This extension module is a front to the Athena and Xlib libraries for Perl GUI programming, originally written by by Dominic Giampaolo < dbg @sgi .com >, then and rewritten for Sx by Frédéric Chauveau< fmc @pasteur .fr >. It's available for FTP from

ftp.pasteur.fr:/pub/Perl/Sx.tar.gz

Tk

This extension module is an object-oriented Perl5 binding to the popular tcl/tk X11 package. However, you need know no TCL to use it! It was written by Malcolm Beattie< mbeattie @sable .ox.ac.uk >. If you are unable to locate it using archie(1L) or a similar tool, you may try retrieving it from /private/Tk-october.tar.gz from Malcolm's machine listed above.