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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 pro-
gramming, networking, and world wide web programming. These strengths make it
especially popular with system administrators and CGI script authors, but mathe-
maticians, 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 documentation you're reading now were all written by vol-
unteers. See the personal note at the end of the README file in the perl source
distribution for more details. See perlhist (new as of 5.005) for Perl's milestone
releases.
In particular, the core development team (known as the Perl Porters) are a rag-tag
band of highly altruistic individuals committed to producing better software for
free than you could hope to purchase for money. You may snoop on pending develop-
ments via the archives at http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/
and http://archive.develooper.com/perl5-porters AT 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 AT 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 com-
mercial 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 current 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.
? If things aren't broken, upgrading perl may break them (or at least issue new
warnings).
? The latest versions of perl have more bug fixes.
? The Perl community is geared toward supporting the most recent releases, so
you'll have an easier time finding help for those.
? Versions prior to perl5.004 had serious security problems with buffer over-
flows, and in some cases have CERT advisories (for instance,
http://www.cert.org/advisories/CA-1997-17.html ).
? 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.
? The immediate, previous releases (i.e. perl5.6.x ) are usually maintained for a
while, although not at the same level as the current releases.
? No one is actively supporting perl4.x. Five years ago it was a dead camel car-
cass (according to this document). Now it's barely a skeleton as its white-
washed bones have fractured or eroded.
? 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).
? There are really two tracks of perl development: a maintenance version 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 versions may include features that don't make it into the sta-
ble versions, 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 signif-
icant 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 references, 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 Salzen-
berg'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 supported 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 reinventing."
--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 flawlessly 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 learning 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 defini-
tion) 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 experimentation flattens the learning curve even more.
Things that make Perl easier to learn: Unix experience, almost any kind of program-
ming experience, an understanding of regular expressions, and the ability to under-
stand 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 for-
get the new perl modules, 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 hope-
fully 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 scripting. For others, it serves as a con-
venient, high-level replacement for most of what they'd program in low-level lan-
guages 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 avail-
able 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 languages dedicated
to a specific problem domain that are simply more convenient for certain kinds of
problems. Perl tries to be all things to all people, but nothing special to any-
one. Examples of specialized languages that come to mind include prolog and mat-
lab.
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 application 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 sys-
tems, 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 limitations given
in the previous statement to some degree, but understand that Perl remains funda-
mentally 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 withstand-
ing. :-)
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 commands--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 com-
piled 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 program-
ming". Consequently, some Perl programmers 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 software 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, simplicity, and power, then the typical man-
ager/supervisor/employee may be persuaded. Regarding using Perl in general, it's
also sometimes helpful 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 conjunc-
tion 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 software and hardware companies throughout the world. In fact, many
Unix vendors now ship Perl by default. Support is usually just a news-posting
away, if you can't find the answer in the comprehensive documentation, 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 compile 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
absolutely need such an old version (released in April 1999) for stability rea-
sons. 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, including perl4, are consid-
ered 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 main-
tenance 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.
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