PERLFAQ1(1) Perl Programmers Reference Guide PERLFAQ1(1)NAMEperlfaq1 - General Questions About Perl
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 utili-
ties, 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 documentation 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
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
developments via the archives at
http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/ and
http://archive.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
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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 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 prob-
lems with buffer overflows, 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 carcass (according to this docu-
ment). Now it's barely a skeleton as its whitewashed
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
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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 stable 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 significant differ-
ences 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 struc-
tures, 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/
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What is perl6?
At The Second O'Reilly Open Source Software Convention,
Larry Wall announced Perl6 development would begin in ear-
nest. Perl6 was an oft used term for Chip Salzenberg's pro-
ject to rewrite Perl in C++ named Topaz. However, Topaz pro-
vided 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 possi-
ble 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 learn-
ing. It looks like most programming languages you're likely
to have experience with, so if you've ever written a C pro-
gram, an awk script, a shell script, or even a BASIC pro-
gram, 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).
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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 intermedi-
ate 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 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
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 start-
ing 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 scripting. 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.
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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 prob-
lems. Perl tries to be all things to all people, but noth-
ing 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 exist-
ing application written in another language that's all done
(and done well), or you have an application language specif-
ically 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 limitations 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 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 imple-
mentation 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."
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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 pro-
grams 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 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
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and utilizing Perl constructs, functionality, simplicity,
and power, then the typical manager/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 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 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 ver-
sion 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 over-
flow problems that went into the 5.004 release. All
releases prior to that, including 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
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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 pro-
grams for fun or for profit as you see fit. A simple com-
ment in the code giving credit to the FAQ would be courteous
but is not required.
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