path: root/pod/perlmod.pod

=head1 NAME

perlmod - Perl modules (packages)

=head1 DESCRIPTION

=head2 Packages

Perl provides a mechanism for alternative namespaces to protect packages
from stomping on each other's variables.  In fact, apart from certain
magical variables, there's really no such thing as a global variable in
Perl.  The package statement declares the compilation unit as being in the
given namespace.  The scope of the package declaration is from the
declaration itself through the end of the enclosing block (the same scope
as the local() operator).  All further unqualified dynamic identifiers
will be in this namespace.  A package statement only affects dynamic
variables--including those you've used local() on--but I<not> lexical
variables created with my().  Typically it would be the first declaration
in a file to be included by the C<require> or C<use> operator.  You can
switch into a package in more than one place; it merely influences which
symbol table is used by the compiler for the rest of that block.  You can
refer to variables and filehandles in other packages by prefixing the
identifier with the package name and a double colon:
C<$Package::Variable>.  If the package name is null, the C<main> package
is assumed.  That is, C<$::sail> is equivalent to C<$main::sail>.

(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 B<emacs> macros.
It also makes C++ programmers feel like they know what's going on.)

Packages may be nested inside other packages: C<$OUTER::INNER::var>.  This
implies nothing about the order of name lookups, however.  All symbols
are either local to the current package, or must be fully qualified
from the outer package name down.  For instance, there is nowhere
within package C<OUTER> that C<$INNER::var> refers to C<$OUTER::INNER::var>.
It would treat package C<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 C<main>,
including all of the punctuation variables like $_.  In addition, the
identifiers STDIN, STDOUT, STDERR, ARGV, ARGVOUT, ENV, INC and SIG are
forced to be in package C<main>, even when used for other purposes than
their built-in one.  Note also that, if you have a package called C<m>,
C<s> or C<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.
$_ is still global though.)

Eval()ed strings are compiled in the package in which the eval() was
compiled.  (Assignments to C<$SIG{}>, however, assume the signal
handler specified is in the C<main> package.  Qualify the signal handler
name if you wish to have a signal handler in a package.)  For an
example, examine F<perldb.pl> in the Perl library.  It initially switches
to the C<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 C<main> package to evaluate various
expressions in the context of the C<main> package (or wherever you came
from).  See L<perldebug>.

See L<perlsub> for other scoping issues related to my() and local(), 
or L<perlref> regarding closures.

=head2 Symbol Tables

The symbol table for a package happens to be stored in the associative
array of that name appended with two colons.  The main symbol table's
name is thus C<%main::>, or C<%::> for short.  Likewise symbol table for
the nested package mentioned earlier is named C<%OUTER::INNER::>.

The value in each entry of the associative array is what you are referring
to when you use the C<*name> typeglob 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:

    local(*main::foo) = *main::bar; local($main::{'foo'}) =
    $main::{'bar'};

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

   package dumpvar;
   sub main::dumpvar {
       ($package) = @_;
       local(*stab) = eval("*${package}::");
       while (($key,$val) = each(%stab)) {
	   local(*entry) = $val;
	   if (defined $entry) {
	       print "\$$key = '$entry'\n";
	   }

	   if (defined @entry) {
	       print "\@$key = (\n";
	       foreach $num ($[ .. $#entry) {
		   print "  $num\t'",$entry[$num],"'\n";
	       }
	       print ")\n";
	   }

	   if ($key ne "${package}::" && defined %entry) {
	       print "\%$key = (\n";
	       foreach $key (sort keys(%entry)) {
		   print "  $key\t'",$entry{$key},"'\n";
	       }
	       print ")\n";
	   }
       }
   }

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

Assignment to a typeglob performs an aliasing operation, i.e.,

    *dick = *richard;

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

    *dick = \$richard;

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

This mechanism may be used to pass and return cheap references
into or from subroutines if you won't want to copy the whole
thing.

    %some_hash = ();
    *some_hash = fn( \%another_hash );
    sub fn {
	local *hashsym = shift;
	# now use %hashsym normally, and you
	# will affect the caller's %another_hash
	my %nhash = (); # do what you want
	return \%nhash; 
    }

On return, the reference wil overwrite the hash slot in the
symbol table specified by the *some_hash typeglob.  This
is a somewhat tricky way of passing around references cheaply
when you won't want to have to remember to dereference variables
explicitly.

Another use of symbol tables is for making "constant"  scalars.

    *PI = \3.14159265358979;

Now you cannot alter $PI, which is probably a good thing all in all.

You can say C<*foo{PACKAGE}> and C<*foo{NAME}> to find out what name and
package the *foo symbol table entry comes from.  This may be useful
in a subroutine which is passed typeglobs as arguments

    sub identify_typeglob {
        my $glob = shift;
        print 'You gave me ', *{$glob}{PACKAGE}, '::', *{$glob}{NAME}, "\n";
    }
    identify_typeglob *foo;
    identify_typeglob *bar::baz;

This prints

    You gave me main::foo
    You gave me bar::baz

The *foo{THING} notation can also be used to obtain references to the
individual elements of *foo, see L<perlref>.

=head2 Package Constructors and Destructors

There are two special subroutine definitions that function as package
constructors and destructors.  These are the C<BEGIN> and C<END>
routines.  The C<sub> is optional for these routines.

A C<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 C<BEGIN> blocks within a
file--they will execute in order of definition.  Because a C<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 C<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 C<END> blocks within a file--they will execute in reverse
order of definition; that is: last in, first out (LIFO).

Inside an C<END> subroutine C<$?> contains the value that the script is
going to pass to C<exit()>.  You can modify C<$?> to change the exit
value of the script.  Beware of changing C<$?> by accident (eg, by
running something via C<system>).

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

=head2 Perl Classes

There is no special class syntax in Perl, but a package may function
as a class if it provides subroutines that function as methods.  Such a
package may also derive some of its methods from another class package
by listing the other package name in its @ISA array.  

For more on this, see L<perlobj>.

=head2 Perl Modules

A module is just a package that is defined in a library file of
the same name, and is designed to be reusable.  It may do this by
providing a mechanism for exporting some of its symbols into the symbol
table of any package using it.  Or it may function as a class
definition and make its semantics available implicitly through method
calls on the class and its objects, without explicit exportation of any
symbols.  Or it can do a little of both.

For example, to start a normal module called Fred, create
a file called Fred.pm and put this at the start of it:

    package Fred;
    use Exporter ();
    @ISA       = qw(Exporter);
    @EXPORT    = qw(func1 func2);
    @EXPORT_OK = qw($sally @listabob %harry func3);

Then go on to declare and use your variables in functions
without any qualifications.
See L<Exporter> and the I<Perl Modules File> for details on 
mechanics and style issues in module creation.

Perl modules are included into your program by saying

    use Module;

or

    use Module LIST;

This is exactly equivalent to

    BEGIN { require "Module.pm"; import Module; }

or

    BEGIN { require "Module.pm"; import Module LIST; }

As a special case

    use Module ();

is exactly equivalent to

    BEGIN { require "Module.pm"; }

All Perl module files have the extension F<.pm>.  C<use> assumes this so
that you don't have to spell out "F<Module.pm>" in quotes.  This also
helps to differentiate new modules from old F<.pl> and F<.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 C<use> statement implies a C<BEGIN> block, the importation
of semantics happens at the moment the C<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 C<require>
instead of C<use>.  With require you can get into this problem:

    require Cwd;		# make Cwd:: accessible
    $here = Cwd::getcwd();	

    use Cwd;			# import names from Cwd:: 
    $here = getcwd();

    require Cwd;	    	# make Cwd:: accessible
    $here = getcwd(); 		# oops! no main::getcwd()

In general C<use Module ();> is recommended over C<require Module;>.

Perl packages may be nested inside other package names, so we can have
package names containing C<::>.  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,
C<Text::Soundex>, then its definition is actually found in the library
file F<Text/Soundex.pm>.

Perl modules always have a F<.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 F<.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 C<use POSIX> to get it all.

For more information on writing extension modules, see L<perlxs>
and L<perlguts>.

=head1 NOTE

Perl does not enforce private and public parts of its modules as you may
have been used to in other languages like C++, Ada, or Modula-17.  Perl
doesn't have an infatuation with enforced privacy.  It would prefer
that you stayed out of its living room because you weren't invited, not
because it has a shotgun.

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 C<use RedefineTheWorld> that
you're redefining the world and willing to take the consequences.

=head1 THE PERL MODULE LIBRARY

A number of modules are included the the Perl distribution.  These are
described below, and all end in F<.pm>.  You may also discover files in 
the library directory that end in either F<.pl> or F<.ph>.  These are old
libraries supplied so that old programs that use them still run.  The
F<.pl> files will all eventually be converted into standard modules, and
the F<.ph> files made by B<h2ph> will probably end up as extension modules
made by B<h2xs>.  (Some F<.ph> values may already be available through the
POSIX module.)  The B<pl2pm> file in the distribution may help in your
conversion, but it's just a mechanical process, so is far from bulletproof.

=head2 Pragmatic Modules

They work somewhat like pragmas in that they tend to affect the compilation of
your program, and thus will usually only work well when used within a
C<use>, or C<no>.  Most of these are locally scoped, so an inner BLOCK
may countermand any of these by saying:

    no integer;
    no strict 'refs';

which lasts until the end of that BLOCK.

Unlike the pragrmas that effect the C<$^H> hints variable, the C<use
vars> and C<use subs> declarations are not BLOCK-scoped.  They allow
you to pre-declare a variables or subroutines within a particular
<I>file</I> rather than just a block.  Such declarations are effective
for the entire file for which they were declared.  You cannot rescind
them with C<no vars> or C<no subs>.

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

=over 12

=item diagnostics

force verbose warning diagnostics

=item integer

compute arithmetic in integer instead of double

=item less

request less of something from the compiler

=item lib

manipulate @INC at compile time

=item ops

restrict unsafe operations when compiling

=item overload

package for overloading perl operations

=item sigtrap

enable simple signal handling

=item strict

restrict unsafe constructs

=item subs

predeclare sub names

=item vars

predeclare global variable names

=back

=head2 Standard Modules

Standard, bundled modules are all expected to behave in a well-defined
manner with respect to namespace pollution because they use the
Exporter module.  See their own documentation for details.

=over 12

=item AnyDBM_File

provide framework for multiple DBMs

=item AutoLoader

load functions only on demand

=item AutoSplit

split a package for autoloading

=item Benchmark

benchmark running times of code

=item Carp

warn of errors (from perspective of caller)

=item Config

access Perl configuration information

=item Cwd

get pathname of current working directory

=item DB_File

access to Berkeley DB

=item Devel::SelfStubber

generate stubs for a SelfLoading module

=item DirHandle

supply object methods for directory handles

=item DynaLoader

Dynamically load C libraries into Perl code

=item English

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

=item Env

import environment variables

=item Exporter

implements default import method for modules

=item ExtUtils::Embed

Utilities for embedding Perl in C/C++ applications

=item ExtUtils::Install

install files from here to there

=item ExtUtils::Liblist

determine libraries to use and how to use them

=item ExtUtils::MakeMaker

create an extension Makefile

=item ExtUtils::Manifest

utilities to write and check a MANIFEST file

=item ExtUtils::Miniperl

write the C code for perlmain.c

=item ExtUtils::Mkbootstrap

make a bootstrap file for use by DynaLoader

=item ExtUtils::Mksymlists

write linker options files for dynamic extension

=item ExtUtils::MM_OS2

methods to override UN*X behaviour in ExtUtils::MakeMaker

=item ExtUtils::MM_Unix

methods used by ExtUtils::MakeMaker

=item ExtUtils::MM_VMS

methods to override UN*X behaviour in ExtUtils::MakeMaker

=item ExtUtils::testlib

add blib/* directories to @INC

=item Fatal

replace functions with equivalents which succeed or die

=item Fcntl

load the C Fcntl.h defines

=item File::Basename

parse file specifications

=item FileCache

keep more files open than the system permits

=item File::CheckTree

run many filetest checks on a tree

=item File::Copy

Copy files or filehandles

=item File::Find

traverse a file tree

=item FileHandle

supply object methods for filehandles

=item File::Path

create or remove a series of directories

=item FindBin

locate directory of original perl script

=item GDBM_File

access to the gdbm library.

=item Getopt::Long

extended processing of command line options

=item Getopt::Std

process single-character switches with switch clustering

=item I18N::Collate

compare 8-bit scalar data according to the current locale

=item IO

load various IO modules

=item IO::File

supply object methods for filehandles

=item IO::Handle

supply object methods for I/O handles

=item IO::Pipe

supply object methods for pipes

=item IO::Seekable

supply seek based methods for I/O objects

=item IO::Select

OO interface to the select system call

=item IO::Socket

object interface to socket communications

=item IPC::Open2

open a process for both reading and writing

=item IPC::Open3

open a process for reading, writing, and error handling

=item Math::BigFloat

arbitrary length float math package

=item Math::BigInt

arbitrary size integer math package

=item Math::Complex

complex numbers and associated mathematical functions

=item NDBM_File

tied access to ndbm files

=item Net::Ping

check a host for upness

=item Opcode

disable named opcodes when compiling perl code

=item Pod::Text

convert POD data to formatted ASCII text

=item POSIX

interface to IEEE Std 1003.1

=item Safe

compile and execute code in restricted compartments

=item SDBM_File

tied access to sdbm files

=item Search::Dict

search for key in dictionary file

=item SelectSaver

save and restore selected file handle

=item SelfLoader

load functions only on demand

=item Shell

run shell commands transparently within perl

=item Socket

load the C socket.h defines and structure manipulators

=item Symbol

manipulate Perl symbols and their names

=item Sys::Hostname

try every conceivable way to get hostname

=item Sys::Syslog

interface to the UNIX syslog(3) calls

=item Term::Cap

Perl termcap interface

=item Term::Complete

word completion module

=item Term::ReadLine

interface to various readline packages. 

=item Test::Harness

run perl standard test scripts with statistics

=item Text::Abbrev

create an abbreviation table from a list

=item Text::ParseWords

parse text into an array of tokens

=item Text::Soundex

implementation of the Soundex Algorithm as Described by Knuth

=item Text::Tabs

expand and unexpand tabs per the unix expand(1) and unexpand(1)

=item Text::Wrap

line wrapping to form simple paragraphs

=item Tie::Hash

base class definitions for tied hashes

=item Tie::Scalar

base class definitions for tied scalars

=item Tie::SubstrHash

fixed-table-size, fixed-key-length hashing

=item Time::Local

efficiently compute time from local and GMT time

=item UNIVERSAL

base class for ALL classes (blessed references)

=back

To find out I<all> the modules installed on your system, including
those without documentation or outside the standard release, do this:

    find `perl -e 'print "@INC"'` -name '*.pm' -print

They should all have their own documentation installed and accessible via
your system man(1) command.  If that fails, try the I<perldoc> program.

=head2 Extension Modules

Extension modules are written in C (or a mix of Perl and C) and get
dynamically loaded into Perl if and when you need them.  Supported
extension modules include the Socket, Fcntl, and POSIX modules.

Many popular C extension modules 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.

=head1 CPAN

CPAN stands for the Comprehensive Perl Archive Network.  This is a globally
replicated collection of all known Perl materials, including hundreds 
of unbundled modules.  Here are the major categories of modules:

=over

=item *
Language Extensions and Documentation Tools 

=item *
Development Support

=item *
Operating System Interfaces

=item *
Networking, Device Control (modems) and InterProcess Communication

=item *
Data Types and Data Type Utilities

=item *
Database Interfaces

=item *
User Interfaces

=item *
Interfaces to / Emulations of Other Programming Languages

=item *
File Names, File Systems and File Locking (see also File Handles)

=item *
String Processing, Language Text Processing, Parsing and Searching

=item *
Option, Argument, Parameter and Configuration File Processing

=item *
Internationalization and Locale

=item *
Authentication, Security and Encryption

=item *
World Wide Web, HTML, HTTP, CGI, MIME

=item *
Server and Daemon Utilities

=item *
Archiving and Compression

=item *
Images, Pixmap and Bitmap Manipulation, Drawing and Graphing

=item *
Mail and Usenet News

=item *
Control Flow Utilities (callbacks and exceptions etc)

=item *
File Handle and Input/Output Stream Utilities

=item *
Miscellaneous Modules

=back

The registered CPAN sites as of this writing include the following.
You should try to choose one close to you:

=over

=item *
ftp://ftp.sterling.com/programming/languages/perl/

=item *
ftp://ftp.sedl.org/pub/mirrors/CPAN/

=item *
ftp://ftp.uoknor.edu/mirrors/CPAN/

=item *
ftp://ftp.delphi.com/pub/mirrors/packages/perl/CPAN/

=item *
ftp://uiarchive.cso.uiuc.edu/pub/lang/perl/CPAN/

=item *
ftp://ftp.cis.ufl.edu/pub/perl/CPAN/

=item *
ftp://ftp.switch.ch/mirror/CPAN/

=item *
ftp://ftp.sunet.se/pub/lang/perl/CPAN/

=item *
ftp://ftp.ci.uminho.pt/pub/lang/perl/

=item *
ftp://ftp.cs.ruu.nl/pub/PERL/CPAN/

=item *
ftp://ftp.demon.co.uk/pub/mirrors/perl/CPAN/

=item *
ftp://ftp.rz.ruhr-uni-bochum.de/pub/programming/languages/perl/CPAN/

=item *
ftp://ftp.leo.org/pub/comp/programming/languages/perl/CPAN/

=item *
ftp://ftp.pasteur.fr/pub/computing/unix/perl/CPAN/

=item *
ftp://ftp.ibp.fr/pub/perl/CPAN/

=item *
ftp://ftp.funet.fi/pub/languages/perl/CPAN/

=item *
ftp://ftp.tekotago.ac.nz/pub/perl/CPAN/

=item *
ftp://ftp.mame.mu.oz.au/pub/perl/CPAN/

=item *
ftp://coombs.anu.edu.au/pub/perl/

=item *
ftp://dongpo.math.ncu.edu.tw/perl/CPAN/

=item *
ftp://ftp.lab.kdd.co.jp/lang/perl/CPAN/

=item *
ftp://ftp.is.co.za/programming/perl/CPAN/

=back

For an up-to-date listing of CPAN sites, 
see F<http://www.perl.com/perl/CPAN> or F<ftp://ftp.perl.com/perl/>.

=head1 Modules: Creation, Use and Abuse

(The following section is borrowed directly from Tim Bunce's modules
file, available at your nearest CPAN site.)

Perl 5 implements a class using a package, but the presence of a
package doesn't imply the presence of a class.  A package is just a
namespace.  A class is a package that provides subroutines that can be
used as methods.  A method is just a subroutine that expects, as its
first argument, either the name of a package (for "static" methods),
or a reference to something (for "virtual" methods).

A module is a file that (by convention) provides a class of the same
name (sans the .pm), plus an import method in that class that can be
called to fetch exported symbols.  This module may implement some of
its methods by loading dynamic C or C++ objects, but that should be
totally transparent to the user of the module.  Likewise, the module
might set up an AUTOLOAD function to slurp in subroutine definitions on
demand, but this is also transparent.  Only the .pm file is required to
exist.

=head2 Guidelines for Module Creation

=over 4

=item Do similar modules already exist in some form?

If so, please try to reuse the existing modules either in whole or
by inheriting useful features into a new class.  If this is not
practical try to get together with the module authors to work on
extending or enhancing the functionality of the existing modules.
A perfect example is the plethora of packages in perl4 for dealing
with command line options.

If you are writing a module to expand an already existing set of
modules, please coordinate with the author of the package.  It
helps if you follow the same naming scheme and module interaction
scheme as the original author.

=item Try to design the new module to be easy to extend and reuse.

Use blessed references.  Use the two argument form of bless to bless
into the class name given as the first parameter of the constructor,
e.g.:

 sub new { 
	my $class = shift;
	return bless {}, $class;
 }

or even this if you'd like it to be used as either a static
or a virtual method.

 sub new { 
	my $self  = shift;
	my $class = ref($self) || $self;
	return bless {}, $class;
 }

Pass arrays as references so more parameters can be added later
(it's also faster).  Convert functions into methods where
appropriate.  Split large methods into smaller more flexible ones.
Inherit methods from other modules if appropriate.

Avoid class name tests like: C<die "Invalid" unless ref $ref eq 'FOO'>.
Generally you can delete the "C<eq 'FOO'>" part with no harm at all.
Let the objects look after themselves! Generally, avoid hardwired
class names as far as possible.

Avoid C<$r-E<gt>Class::func()> where using C<@ISA=qw(... Class ...)> and
C<$r-E<gt>func()> would work (see L<perlbot> for more details).

Use autosplit so little used or newly added functions won't be a
burden to programs which don't use them. Add test functions to
the module after __END__ either using AutoSplit or by saying:

 eval join('',<main::DATA>) || die $@ unless caller();

Does your module pass the 'empty sub-class' test? If you say
"C<@SUBCLASS::ISA = qw(YOURCLASS);>" your applications should be able
to use SUBCLASS in exactly the same way as YOURCLASS.  For example,
does your application still work if you change:  C<$obj = new YOURCLASS;>
into: C<$obj = new SUBCLASS;> ?

Avoid keeping any state information in your packages. It makes it
difficult for multiple other packages to use yours. Keep state
information in objects.

Always use B<-w>. Try to C<use strict;> (or C<use strict qw(...);>).
Remember that you can add C<no strict qw(...);> to individual blocks
of code which need less strictness. Always use B<-w>. Always use B<-w>!
Follow the guidelines in the perlstyle(1) manual.

=item Some simple style guidelines

The perlstyle manual supplied with perl has many helpful points.

Coding style is a matter of personal taste. Many people evolve their
style over several years as they learn what helps them write and
maintain good code.  Here's one set of assorted suggestions that
seem to be widely used by experienced developers:

Use underscores to separate words.  It is generally easier to read
$var_names_like_this than $VarNamesLikeThis, especially for
non-native speakers of English. It's also a simple rule that works
consistently with VAR_NAMES_LIKE_THIS.

Package/Module names are an exception to this rule. Perl informally
reserves lowercase module names for 'pragma' modules like integer
and strict. Other modules normally begin with a capital letter and
use mixed case with no underscores (need to be short and portable).

You may find it helpful to use letter case to indicate the scope
or nature of a variable. For example:

 $ALL_CAPS_HERE   constants only (beware clashes with perl vars)
 $Some_Caps_Here  package-wide global/static
 $no_caps_here    function scope my() or local() variables

Function and method names seem to work best as all lowercase.
E.g., C<$obj-E<gt>as_string()>.

You can use a leading underscore to indicate that a variable or
function should not be used outside the package that defined it.

=item Select what to export.

Do NOT export method names!

Do NOT export anything else by default without a good reason!

Exports pollute the namespace of the module user.  If you must
export try to use @EXPORT_OK in preference to @EXPORT and avoid
short or common names to reduce the risk of name clashes.

Generally anything not exported is still accessible from outside the
module using the ModuleName::item_name (or C<$blessed_ref-E<gt>method>)
syntax.  By convention you can use a leading underscore on names to
informally indicate that they are 'internal' and not for public use.

(It is actually possible to get private functions by saying:
C<my $subref = sub { ... };  &$subref;>.  But there's no way to call that
directly as a method, since a method must have a name in the symbol
table.)

As a general rule, if the module is trying to be object oriented
then export nothing. If it's just a collection of functions then
@EXPORT_OK anything but use @EXPORT with caution.

=item Select a name for the module.

This name should be as descriptive, accurate and complete as
possible.  Avoid any risk of ambiguity. Always try to use two or
more whole words.  Generally the name should reflect what is special
about what the module does rather than how it does it.  Please use
nested module names to informally group or categorise a module.
A module should have a very good reason not to have a nested name.
Module names should begin with a capital letter.

Having 57 modules all called Sort will not make life easy for anyone
(though having 23 called Sort::Quick is only marginally better :-).
Imagine someone trying to install your module alongside many others.
If in any doubt ask for suggestions in comp.lang.perl.misc.

If you are developing a suite of related modules/classes it's good
practice to use nested classes with a common prefix as this will
avoid namespace clashes. For example:  Xyz::Control, Xyz::View,
Xyz::Model etc. Use the modules in this list as a naming guide.

If adding a new module to a set, follow the original author's
standards for naming modules and the interface to methods in
those modules.

To be portable each component of a module name should be limited to
11 characters. If it might be used on DOS then try to ensure each is
unique in the first 8 characters. Nested modules make this easier.

=item Have you got it right?

How do you know that you've made the right decisions? Have you
picked an interface design that will cause problems later? Have
you picked the most appropriate name? Do you have any questions?

The best way to know for sure, and pick up many helpful suggestions,
is to ask someone who knows. Comp.lang.perl.misc is read by just about
all the people who develop modules and it's the best place to ask.

All you need to do is post a short summary of the module, its
purpose and interfaces. A few lines on each of the main methods is
probably enough. (If you post the whole module it might be ignored
by busy people - generally the very people you want to read it!)

Don't worry about posting if you can't say when the module will be
ready - just say so in the message. It might be worth inviting
others to help you, they may be able to complete it for you!

=item README and other Additional Files.

It's well known that software developers usually fully document the
software they write. If, however, the world is in urgent need of
your software and there is not enough time to write the full
documentation please at least provide a README file containing:

=over 10

=item *
A description of the module/package/extension etc.

=item *
A copyright notice - see below.

=item *
Prerequisites - what else you may need to have.

=item *
How to build it - possible changes to Makefile.PL etc.

=item *
How to install it.

=item *
Recent changes in this release, especially incompatibilities

=item *
Changes / enhancements you plan to make in the future.

=back

If the README file seems to be getting too large you may wish to
split out some of the sections into separate files: INSTALL,
Copying, ToDo etc.

=over 4

=item Adding a Copyright Notice.

How you choose to licence your work is a personal decision.
The general mechanism is to assert your Copyright and then make
a declaration of how others may copy/use/modify your work.

Perl, for example, is supplied with two types of license: The GNU
GPL and The Artistic License (see the files README, Copying and
Artistic).  Larry has good reasons for NOT just using the GNU GPL.

My personal recommendation, out of respect for Larry, Perl and the
perl community at large is to simply state something like:

 Copyright (c) 1995 Your Name. All rights reserved.
 This program is free software; you can redistribute it and/or
 modify it under the same terms as Perl itself.

This statement should at least appear in the README file. You may
also wish to include it in a Copying file and your source files.
Remember to include the other words in addition to the Copyright.

=item Give the module a version/issue/release number.

To be fully compatible with the Exporter and MakeMaker modules you
should store your module's version number in a non-my package
variable called $VERSION.  This should be a valid floating point 
number with at least two digits after the decimal (ie hundredths,
e.g, C<$VERSION = "0.01">).  Don't use a "1.3.2" style version.
See Exporter.pm in Perl5.001m or later for details.

It may be handy to add a function or method to retrieve the number.
Use the number in announcements and archive file names when
releasing the module (ModuleName-1.02.tar.Z).
See perldoc ExtUtils::MakeMaker.pm for details.

=item How to release and distribute a module.

It's good idea to post an announcement of the availability of your
module (or the module itself if small) to the comp.lang.perl.announce
Usenet newsgroup.  This will at least ensure very wide once-off
distribution.

If possible you should place the module into a major ftp archive and
include details of it's location in your announcement.

Some notes about ftp archives: Please use a long descriptive file
name which includes the version number. Most incoming directories
will not be readable/listable, i.e., you won't be able to see your
file after uploading it. Remember to send your email notification
message as soon as possible after uploading else your file may get
deleted automatically. Allow time for the file to be processed
and/or check the file has been processed before announcing its
location.

FTP Archives for Perl Modules:

Follow the instructions and links on

   http://franz.ww.tu-berlin.de/modulelist

or upload to one of these sites: 

   ftp://franz.ww.tu-berlin.de/incoming
   ftp://ftp.cis.ufl.edu/incoming  

and notify upload@franz.ww.tu-berlin.de.

By using the WWW interface you can ask the Upload Server to mirror
your modules from your ftp or WWW site into your own directory on
CPAN!

Please remember to send me an updated entry for the Module list!

=item Take care when changing a released module.

Always strive to remain compatible with previous released versions
(see 2.2 above) Otherwise try to add a mechanism to revert to the
old behaviour if people rely on it. Document incompatible changes.

=back

=back

=head2 Guidelines for Converting Perl 4 Library Scripts into Modules

=over 4

=item There is no requirement to convert anything.

If it ain't broke, don't fix it! Perl 4 library scripts should
continue to work with no problems. You may need to make some minor
changes (like escaping non-array @'s in double quoted strings) but
there is no need to convert a .pl file into a Module for just that.

=item Consider the implications.

All the perl applications which make use of the script will need to
be changed (slightly) if the script is converted into a module.  Is
it worth it unless you plan to make other changes at the same time?

=item Make the most of the opportunity.

If you are going to convert the script to a module you can use the
opportunity to redesign the interface. The 'Guidelines for Module
Creation' above include many of the issues you should consider.

=item The pl2pm utility will get you started.

This utility will read *.pl files (given as parameters) and write
corresponding *.pm files. The pl2pm utilities does the following:

=over 10

=item *
Adds the standard Module prologue lines

=item *
Converts package specifiers from ' to ::

=item *
Converts die(...) to croak(...)

=item *
Several other minor changes

=back

Being a mechanical process pl2pm is not bullet proof. The converted
code will need careful checking, especially any package statements.
Don't delete the original .pl file till the new .pm one works!

=back

=head2 Guidelines for Reusing Application Code

=over 4

=item Complete applications rarely belong in the Perl Module Library.

=item Many applications contain some perl code which could be reused.

Help save the world! Share your code in a form that makes it easy
to reuse.

=item Break-out the reusable code into one or more separate module files.

=item Take the opportunity to reconsider and redesign the interfaces.

=item In some cases the 'application' can then be reduced to a small

fragment of code built on top of the reusable modules. In these cases
the application could invoked as:

     perl -e 'use Module::Name; method(@ARGV)' ...
or   
     perl -mModule::Name ...    (in perl5.002)

=back