PERLFAQ1(1) Perl Programmers Reference Guide PERLFAQ1(1)NAMEperlfaq1 - General Questions About Perl ($Revision: 1.20
$, $Date: 1999/01/08 04:22:09 $)
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 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 the
perlhist manpage (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
nntp://news.perl.com/perl.porters-gw/ and the Deja News
archive at http://www.dejanews.com/ using the
perl.porters-gw newsgroup, or you can subscribe to the
mailing list by sending perl5-porters-request@perl.org a
subscription request.
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.
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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 5.005_02 (although 5.004_04 is still
supported). The most cutting-edge development release is
5.005_54. 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
for 5.005_02 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
the section on Perl4 to Perl5 Traps in the perltrap
manpage.
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 the perlhist manpage for a history of Perl revisions.
What is perl6?
Perl6 is a semi-jocular reference to the Topaz project.
Headed by Chip Salzenberg, Topaz is yet-another ground-up
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rewrite of the current release of Perl, one whose major
goal is to create a more maintainable core than found in
release 5. Written in nominally portable C++, Topaz hopes
to maintain 100% source-compatibility with previous
releases of Perl but to run significantly faster and
smaller. The Topaz team hopes to provide an XS
compatibility interface to allow most XS modules to work
unchanged, albeit perhaps without the efficiency that the
new interface uowld allow. New features in Topaz are as
yet undetermined, and will be addressed once compatibility
and performance goals are met.
If you are a hard-working C++ wizard with a firm command
of Perl's internals, and you would like to work on the
project, send a request to perl6-porters-request@perl.org
to subscribe to the Topaz mailing list.
There is no ETA for Topaz. It is expected to be several
years before it achieves enough robustness, compatibility,
portability, and performance to replace perl5 for ordinary
use by mere mortals.
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
an C program, an awk script, a shell script, or even BASIC
program, you're already part way 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
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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 the perlembed manpage.
That said, there will always be small, focused, special-
purpose languages dedicated to a specific problem domain
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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 isn't really an
acronym, aprocryphal 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
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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.perl.com/CPAN/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.perl.com/CPAN/misc/lwall-quotes.txt.gz .
Newer examples can be found by perusing Larry's postings:
http://x1.dejanews.com/dnquery.xp?QRY=*&DBS=2&ST=PS&defaultOp=AND&LNG=ALL&format=terse&showsort=date&maxhits=100&subjects=&groups=&authors=larry@*wall.org&fromdate=&todate=
How can I convince my sysadmin/supervisor/employees to use
version (5/5.005/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
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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,
as 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, and 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/or hardware companies throughout the
world. In fact, many Unix vendors now ship Perl by
default, and 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 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. That probably means running the 5.005
release, although 5.004 isn't that bad. Several important
bugs were fixed from the 5.000 through 5.003 versions,
though, so try upgrading past them if possible.
Of particular note is the massive bughunt 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.
AUTHOR AND COPYRIGHT
Copyright (c) 1997-1999 Tom Christiansen and Nathan
Torkington. All rights reserved.
When included as an integrated part of the Standard
Distribution of Perl or of its documentation (printed or
otherwise), this work is covered under Perl's Artistic
Licence. For separate distributions of all or part of
this FAQ outside of that, see the perlfaq manpage.
Irrespective of its distribution, all code examples here
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are 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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