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PERLFAQ1(1) Perl Programmers Reference Guide PERLFAQ1(1)
NAME
perlfaq1 - General Questions About Perl ($Revision: 1.19 $, $Date:
2005/12/31 00:54:37 $)
DESCRIPTION
This section of the FAQ answers very general, high-level questions
about Perl.
What is Perl?
Perl is a high-level programming language with an eclectic heritage
written by Larry Wall and a cast of thousands. It derives from the
ubiquitous C programming language and to a lesser extent from sed, awk,
the Unix shell, and at least a dozen other tools and languages. Perl's
process, file, and text manipulation facilities make it particularly
well-suited for tasks involving quick prototyping, system utilities,
software tools, system management tasks, database access, graphical
programming, networking, and world wide web programming. These
strengths make it especially popular with system administrators and CGI
script authors, but mathematicians, geneticists, journalists, and even
managers also use Perl. Maybe you should, too.
Who supports Perl? Who develops it? Why is it free?
The original culture of the pre-populist Internet and the deeply-held
beliefs of Perl's author, Larry Wall, gave rise to the free and open
distribution policy of perl. Perl is supported by its users. The
core, the standard Perl library, the optional modules, and the documen-
tation you're reading now were all written by volunteers. See the per-
sonal note at the end of the README file in the perl source distribu-
tion for more details. See perlhist (new as of 5.005) for Perl's mile-
stone releases.
In particular, the core development team (known as the Perl Porters)
are a rag-tag band of highly altruistic individuals committed to pro-
ducing better software for free than you could hope to purchase for
money. You may snoop on pending developments via the archives at
http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/ and http://ar-
chive.develooper.com/perl5-porters@perl.org/ or the news gateway
nntp://nntp.perl.org/perl.perl5.porters or its web interface at
http://nntp.perl.org/group/perl.perl5.porters , or read the faq at
http://simon-cozens.org/writings/p5p-faq , or you can subscribe to the
mailing list by sending perl5-porters-request@perl.org a subscription
request (an empty message with no subject is fine).
While the GNU project includes Perl in its distributions, there's no
such thing as "GNU Perl". Perl is not produced nor maintained by the
Free Software Foundation. Perl's licensing terms are also more open
than GNU software's tend to be.
You can get commercial support of Perl if you wish, although for most
users the informal support will more than suffice. See the answer to
"Where can I buy a commercial version of perl?" for more information.
Which version of Perl should I use?
(contributed by brian d foy)
There is often a matter of opinion and taste, and there isn't any one
answer that fits anyone. In general, you want to use either the cur-
rent stable release, or the stable release immediately prior to that
one. Currently, those are perl5.8.x and perl5.6.x, respectively.
Beyond that, you have to consider several things and decide which is
best for you.
o If things aren't broken, upgrading perl may break them (or at least
issue new warnings).
o The latest versions of perl have more bug fixes.
o The Perl community is geared toward supporting the most recent
releases, so you'll have an easier time finding help for those.
o Versions prior to perl5.004 had serious security problems with
buffer overflows, and in some cases have CERT advisories (for
instance, http://www.cert.org/advisories/CA-1997-17.html ).
o The latest versions are probably the least deployed and widely
tested, so you may want to wait a few months after their release
and see what problems others have if you are risk averse.
o The immediate, previous releases (i.e. perl5.6.x ) are usually
maintained for a while, although not at the same level as the cur-
rent releases.
o No one is actively supporting perl4.x. Five years ago it was a
dead camel carcass (according to this document). Now it's barely a
skeleton as its whitewashed bones have fractured or eroded.
o There is no perl6.x for the next couple of years. Stay tuned, but
don't worry that you'll have to change major versions of Perl soon
(i.e. before 2006).
o There are really two tracks of perl development: a maintenance ver-
sion and an experimental version. The maintenance versions are
stable, and have an even number as the minor release (i.e.
perl5.8.x, where 8 is the minor release). The experimental ver-
sions may include features that don't make it into the stable ver-
sions, and have an odd number as the minor release (i.e. perl5.9.x,
where 9 is the minor release).
What are perl4, perl5, or perl6?
(contributed by brian d foy)
In short, perl4 is the past, perl5 is the present, and perl6 is the
future.
The number after perl (i.e. the 5 after perl5) is the major release of
the perl interpreter as well as the version of the language. Each
major version has significant differences that earlier versions cannot
support.
The current major release of Perl is perl5, and was released in 1994.
It can run scripts from the previous major release, perl4 (March 1991),
but has significant differences. It introduced the concept of refer-
ences, complex data structures, and modules. The perl5 interpreter was
a complete re-write of the previous perl sources.
Perl6 is the next major version of Perl, but it's still in development
in both its syntax and design. The work started in 2002 and is still
ongoing. Many of the most interesting features have shown up in the
latest versions of perl5, and some perl5 modules allow you to use some
perl6 syntax in your programs. You can learn more about perl6 at
http://dev.perl.org/perl6/ .
See perlhist for a history of Perl revisions.
What is Ponie?
At The O'Reilly Open Source Software Convention in 2003, Artur Bergman,
Fotango, and The Perl Foundation announced a project to run perl5 on
the Parrot virtual machine named Ponie. Ponie stands for Perl On New
Internal Engine. The Perl 5.10 language implementation will be used
for Ponie, and there will be no language level differences between
perl5 and ponie. Ponie is not a complete rewrite of perl5.
For more details, see http://www.poniecode.org/
What is perl6?
At The Second O'Reilly Open Source Software Convention, Larry Wall
announced Perl6 development would begin in earnest. Perl6 was an oft
used term for Chip Salzenberg's project to rewrite Perl in C++ named
Topaz. However, Topaz provided valuable insights to the next version of
Perl and its implementation, but was ultimately abandoned.
If you want to learn more about Perl6, or have a desire to help in the
crusade to make Perl a better place then peruse the Perl6 developers
page at http://dev.perl.org/perl6/ and get involved.
Perl6 is not scheduled for release yet, and Perl5 will still be sup-
ported for quite awhile after its release. Do not wait for Perl6 to do
whatever you need to do.
"We're really serious about reinventing everything that needs reinvent-
ing." --Larry Wall
How stable is Perl?
Production releases, which incorporate bug fixes and new functionality,
are widely tested before release. Since the 5.000 release, we have
averaged only about one production release per year.
Larry and the Perl development team occasionally make changes to the
internal core of the language, but all possible efforts are made toward
backward compatibility. While not quite all perl4 scripts run flaw-
lessly under perl5, an update to perl should nearly never invalidate a
program written for an earlier version of perl (barring accidental bug
fixes and the rare new keyword).
Is Perl difficult to learn?
No, Perl is easy to start learning--and easy to keep learning. It
looks like most programming languages you're likely to have experience
with, so if you've ever written a C program, an awk script, a shell
script, or even a BASIC program, you're already partway there.
Most tasks only require a small subset of the Perl language. One of
the guiding mottos for Perl development is "there's more than one way
to do it" (TMTOWTDI, sometimes pronounced "tim toady"). Perl's learn-
ing curve is therefore shallow (easy to learn) and long (there's a
whole lot you can do if you really want).
Finally, because Perl is frequently (but not always, and certainly not
by definition) an interpreted language, you can write your programs and
test them without an intermediate compilation step, allowing you to
experiment and test/debug quickly and easily. This ease of experimen-
tation flattens the learning curve even more.
Things that make Perl easier to learn: Unix experience, almost any kind
of programming experience, an understanding of regular expressions, and
the ability to understand other people's code. If there's something
you need to do, then it's probably already been done, and a working
example is usually available for free. Don't forget the new perl mod-
ules, either. They're discussed in Part 3 of this FAQ, along with
CPAN, which is discussed in Part 2.
How does Perl compare with other languages like Java, Python, REXX,
Scheme, or Tcl?
Favorably in some areas, unfavorably in others. Precisely which areas
are good and bad is often a personal choice, so asking this question on
Usenet runs a strong risk of starting an unproductive Holy War.
Probably the best thing to do is try to write equivalent code to do a
set of tasks. These languages have their own newsgroups in which you
can learn about (but hopefully not argue about) them.
Some comparison documents can be found at
http://www.perl.com/doc/FMTEYEWTK/versus/ if you really can't stop
yourself.
Can I do [task] in Perl?
Perl is flexible and extensible enough for you to use on virtually any
task, from one-line file-processing tasks to large, elaborate systems.
For many people, Perl serves as a great replacement for shell script-
ing. For others, it serves as a convenient, high-level replacement for
most of what they'd program in low-level languages like C or C++. It's
ultimately up to you (and possibly your management) which tasks you'll
use Perl for and which you won't.
If you have a library that provides an API, you can make any component
of it available as just another Perl function or variable using a Perl
extension written in C or C++ and dynamically linked into your main
perl interpreter. You can also go the other direction, and write your
main program in C or C++, and then link in some Perl code on the fly,
to create a powerful application. See perlembed.
That said, there will always be small, focused, special-purpose lan-
guages dedicated to a specific problem domain that are simply more con-
venient for certain kinds of problems. Perl tries to be all things to
all people, but nothing special to anyone. Examples of specialized
languages that come to mind include prolog and matlab.
When shouldn't I program in Perl?
When your manager forbids it--but do consider replacing them :-).
Actually, one good reason is when you already have an existing applica-
tion written in another language that's all done (and done well), or
you have an application language specifically designed for a certain
task (e.g. prolog, make).
For various reasons, Perl is probably not well-suited for real-time
embedded systems, low-level operating systems development work like
device drivers or context-switching code, complex multi-threaded
shared-memory applications, or extremely large applications. You'll
notice that perl is not itself written in Perl.
The new, native-code compiler for Perl may eventually reduce the limi-
tations given in the previous statement to some degree, but understand
that Perl remains fundamentally a dynamically typed language, not a
statically typed one. You certainly won't be chastised if you don't
trust nuclear-plant or brain-surgery monitoring code to it. And Larry
will sleep easier, too--Wall Street programs not withstanding. :-)
What's the difference between "perl" and "Perl"?
One bit. Oh, you weren't talking ASCII? :-) Larry now uses "Perl" to
signify the language proper and "perl" the implementation of it, i.e.
the current interpreter. Hence Tom's quip that "Nothing but perl can
parse Perl." You may or may not choose to follow this usage. For
example, parallelism means "awk and perl" and "Python and Perl" look
OK, while "awk and Perl" and "Python and perl" do not. But never write
"PERL", because perl is not an acronym, apocryphal folklore and post-
facto expansions notwithstanding.
Is it a Perl program or a Perl script?
Larry doesn't really care. He says (half in jest) that "a script is
what you give the actors. A program is what you give the audience."
Originally, a script was a canned sequence of normally interactive com-
mands--that is, a chat script. Something like a UUCP or PPP chat
script or an expect script fits the bill nicely, as do configuration
scripts run by a program at its start up, such .cshrc or .ircrc, for
example. Chat scripts were just drivers for existing programs, not
stand-alone programs in their own right.
A computer scientist will correctly explain that all programs are
interpreted and that the only question is at what level. But if you
ask this question of someone who isn't a computer scientist, they might
tell you that a program has been compiled to physical machine code once
and can then be run multiple times, whereas a script must be translated
by a program each time it's used.
Perl programs are (usually) neither strictly compiled nor strictly
interpreted. They can be compiled to a byte-code form (something of a
Perl virtual machine) or to completely different languages, like C or
assembly language. You can't tell just by looking at it whether the
source is destined for a pure interpreter, a parse-tree interpreter, a
byte-code interpreter, or a native-code compiler, so it's hard to give
a definitive answer here.
Now that "script" and "scripting" are terms that have been seized by
unscrupulous or unknowing marketeers for their own nefarious purposes,
they have begun to take on strange and often pejorative meanings, like
"non serious" or "not real programming". Consequently, some Perl pro-
grammers prefer to avoid them altogether.
What is a JAPH?
These are the "just another perl hacker" signatures that some people
sign their postings with. Randal Schwartz made these famous. About
100 of the earlier ones are available from
http://www.cpan.org/misc/japh .
Where can I get a list of Larry Wall witticisms?
Over a hundred quips by Larry, from postings of his or source code, can
be found at http://www.cpan.org/misc/lwall-quotes.txt.gz .
How can I convince my sysadmin/supervisor/employees to use version
5/5.6.1/Perl instead of some other language?
If your manager or employees are wary of unsupported software, or soft-
ware which doesn't officially ship with your operating system, you
might try to appeal to their self-interest. If programmers can be more
productive using and utilizing Perl constructs, functionality, simplic-
ity, and power, then the typical manager/supervisor/employee may be
persuaded. Regarding using Perl in general, it's also sometimes help-
ful to point out that delivery times may be reduced using Perl compared
to other languages.
If you have a project which has a bottleneck, especially in terms of
translation or testing, Perl almost certainly will provide a viable,
quick solution. In conjunction with any persuasion effort, you should
not fail to point out that Perl is used, quite extensively, and with
extremely reliable and valuable results, at many large computer soft-
ware and hardware companies throughout the world. In fact, many Unix
vendors now ship Perl by default. Support is usually just a news-post-
ing away, if you can't find the answer in the comprehensive documenta-
tion, including this FAQ.
See http://www.perl.org/advocacy/ for more information.
If you face reluctance to upgrading from an older version of perl, then
point out that version 4 is utterly unmaintained and unsupported by the
Perl Development Team. Another big sell for Perl5 is the large number
of modules and extensions which greatly reduce development time for any
given task. Also mention that the difference between version 4 and
version 5 of Perl is like the difference between awk and C++. (Well,
OK, maybe it's not quite that distinct, but you get the idea.) If you
want support and a reasonable guarantee that what you're developing
will continue to work in the future, then you have to run the supported
version. As of December 2003 that means running either 5.8.2 (released
in November 2003), or one of the older releases like 5.6.2 (also
released in November 2003; a maintenance release to let perl 5.6 com-
pile on newer systems as 5.6.1 was released in April 2001) or 5.005_03
(released in March 1999), although 5.004_05 isn't that bad if you abso-
lutely need such an old version (released in April 1999) for stability
reasons. Anything older than 5.004_05 shouldn't be used.
Of particular note is the massive bug hunt for buffer overflow problems
that went into the 5.004 release. All releases prior to that, includ-
ing perl4, are considered insecure and should be upgraded as soon as
possible.
In August 2000 in all Linux distributions a new security problem was
found in the optional 'suidperl' (not built or installed by default) in
all the Perl branches 5.6, 5.005, and 5.004, see
http://www.cpan.org/src/5.0/sperl-2000-08-05/ Perl maintenance releases
5.6.1 and 5.8.0 have this security hole closed. Most, if not all,
Linux distribution have patches for this vulnerability available, see
http://www.linuxsecurity.com/advisories/ , but the most recommendable
way is to upgrade to at least Perl 5.6.1.
AUTHOR AND COPYRIGHT
Copyright (c) 1997-2006 Tom Christiansen, Nathan Torkington, and other
authors as noted. All rights reserved.
This documentation is free; you can redistribute it and/or modify it
under the same terms as Perl itself.
Irrespective of its distribution, all code examples here are in the
public domain. You are permitted and encouraged to use this code and
any derivatives thereof in your own programs for fun or for profit as
you see fit. A simple comment in the code giving credit to the FAQ
would be courteous but is not required.
perl v5.8.8 2006-01-07 PERLFAQ1(1)
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