perlfaq1 - General Questions About Perl ($Revision: 1.7 $,
$Date: 2004/04/07 21:33:08 $)
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 documentation
you're reading now were all written by volunteers. 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 developments via the archives at
http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/
and http://archive.develooper.com/[email protected]/
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
[email protected] 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?
You should definitely use version 5. Version 4 is old,
limited, and no longer maintained; its last patch (4.036)
was in 1992, long ago and far away. Sure, it's stable,
but so is anything that's dead; in fact, perl4 had been
called a dead, flea-bitten camel carcass. The most recent
production release is 5.8.2 (although 5.005_03 and 5.6.2
are still supported). The most cutting-edge development
release is 5.9. Further references to the Perl language
in this document refer to the production release unless
otherwise specified. There may be one or more official
bug fixes by the time you read this, and also perhaps some
experimental versions on the way to the next release. All
releases prior to 5.004 were subject to buffer overruns, a
grave security issue.
What are perl4 and perl5?
Perl4 and perl5 are informal names for different versions
of the Perl programming language. It's easier to say
"perl5" than it is to say "the 5(.004) release of Perl",
but some people have interpreted this to mean there's a
language called "perl5", which isn't the case. Perl5 is
merely the popular name for the fifth major release (October
1994), while perl4 was the fourth major release (March
1991). There was also a perl1 (in January 1988), a perl2
(June 1988), and a perl3 (October 1989).
The 5.0 release is, essentially, a ground-up rewrite of
the original perl source code from releases 1 through 4.
It has been modularized, object-oriented, tweaked,
trimmed, and optimized until it almost doesn't look like
the old code. However, the interface is mostly the same,
and compatibility with previous releases is very high.
See "Perl4 to Perl5 Traps" in perltrap.
To avoid the "what language is perl5?" confusion, some
people prefer to simply use "perl" to refer to the latest
version of perl and avoid using "perl5" altogether. It's
not really that big a deal, though.
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 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 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 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
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://language.perl.com/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.
That said, there will always be small, focused, specialpurpose
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 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 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 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
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 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 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 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 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 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, 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.linuxse-
curity.com/advisories/ , but the most recommendable way is
to upgrade to at least Perl 5.6.1.
Copyright (c) 1997, 1998, 1999, 2000, 2001 Tom Christiansen
and Nathan Torkington. 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 v5.8.5 2002-11-06 8 [ Back ] |
