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Programming Perl
Programming Perl
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By Larry Wall, Tom Christiansen & Randal L. Schwartz; ISBN 1-56592-149-6, 670 pages.
Second Edition, September 1996.
(See the catalog page for this book.)
Search the text of Programming Perl.
Index
Symbols | A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z
Table of Contents
Copyright © 1999 O'Reilly & Associates . All Rights Reserved.
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Preface
Preface
Perl in a Nutshell
Perl is a language for getting your job done.
Of course, if your job is programming, you can get your job done with any "complete" computer
language, theoretically speaking. But we know from experience that computer languages differ not so
much in what they make possible , but in what they make easy . At one extreme, the so-called "fourth
generation languages" make it easy to do some things, but nearly impossible to do other things. At the
other extreme, certain well known, "industrial-strength" languages make it equally difficult to do almost
everything.
Perl is different. In a nutshell, Perl is designed to make the easy jobs easy, without making the hard jobs
impossible.
And what are these "easy jobs" that ought to be easy? The ones you do every day, of course. You want a
language that makes it easy to manipulate numbers and text, files and directories, computers and
networks, and especially programs. It should be easy to run external programs and scan their output for
interesting tidbits. It should be easy to send those same tidbits off to other programs that can do special
things with them. It should be easy to develop, modify, and debug your own programs too. And, of
course, it should be easy to compile and run your programs, and do it portably, on any modern operating
system.
Perl does all that, and a whole lot more.
Initially designed as a glue language for the UNIX operating system (or any of its myriad variants), Perl
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also runs on numerous other systems, including MS-DOS, VMS, OS/2, Plan 9, Macintosh, and any
variety of Windows you care to mention. It is one of the most portable programming languages available
today. To program C portably, you have to put in all those strange #ifdef markings for different
operating systems. And to program a shell portably, you have to remember the syntax for each operating
system's version of each command, and somehow find the least common denominator that (you hope)
works everywhere. Perl happily avoids both of these problems, while retaining many of the benefits of
both C and shell programming, with some additional magic of its own. Much of the explosive growth of
Perl has been fueled by the hankerings of former UNIX programmers who wanted to take along with
them as much of the "old country" as they could. For them, Perl is the portable distillation of UNIX
culture, an oasis in the wilderness of "can't get there from here". On the other hand, it works in the other
direction, too: Web programmers are often delighted to discover that they can take their scripts from a
Windows machine and run them unchanged on their UNIX servers.
Although Perl is especially popular with systems programmers and Web developers, it also appeals to a
much broader audience. The hitherto well-kept secret is now out: Perl is no longer just for text
processing. It has grown into a sophisticated, general-purpose programming language with a rich
software development environment complete with debuggers, profilers, cross-referencers, compilers,
interpreters, libraries, syntax-directed editors, and all the rest of the trappings of a "real" programming
language. (But don't let that scare you: nothing requires you to go tinkering under the hood.) Perl is being
used daily in every imaginable field, from aerospace engineering to molecular biology, from
computer-assisted design/computer-assisted manufacturing (CAD/CAM) to document processing, from
database manipulation to client-server network management. Perl is used by people who are desperate to
analyze or convert lots of data quickly, whether you're talking DNA sequences, Web pages, or pork belly
futures. Indeed, one of the jokes in the Perl community is that the next big stock market crash will
probably be triggered by a bug in a Perl script. (On the brighter side, any unemployed stock analysts will
still have a marketable skill, so to speak.)
There are many reasons for the success of Perl. It certainly helps that Perl is freely available, and freely
redistributable. But that's not enough to explain the Perl phenomenon, since many freeware packages fail
to thrive. Perl is not just free; it's also fun. People feel like they can be creative in Perl, because they have
freedom of expression: they get to choose what to optimize for, whether that's computer speed or
programmer speed, verbosity or conciseness, readability or maintainability or reusability or portability or
learnability or teachability. You can even optimize for obscurity, if you're entering an Obfuscated Perl
contest.
Perl can give you all these degrees of freedom because it's essentially a language with a split personality.
It's both a very simple language and a very rich language. It has taken good ideas from nearly
everywhere, and installed them into an easy-to-use mental framework. To those who merely like it, Perl
is the Practical Extraction and Report Language . To those who love it, Perl is the Pathologically
Eclectic Rubbish Lister . And to the minimalists in the crowd, Perl seems like a pointless exercise in
redundancy. But that's okay. The world needs a few reductionists (mainly as physicists). Reductionists
like to take things apart. The rest of us are just trying to get it together.
Perl is in many ways a simple language. You don't have to know many special incantations to compile a
Perl program - you can just execute it like a shell script. The types and structures used by Perl are easy to
use and understand. Perl doesn't impose arbitrary limitations on your data - your strings and arrays can
grow as large as they like (so long as you have memory), and they're designed to scale well as they grow.
Instead of forcing you to learn new syntax and semantics, Perl borrows heavily from other languages you
may already be familiar with (such as C, and sed , and awk , and English, and Greek). In fact, just about
any programmer can read a well-written piece of Perl code and have some idea of what it does.
Most important, you don't have to know everything there is to know about Perl before you can write
useful programs. You can learn Perl "small end first". You can program in Perl Baby-Talk, and we
promise not to laugh. Or more precisely, we promise not to laugh any more than we'd giggle at a child's
creative way of putting things. Many of the ideas in Perl are borrowed from natural language, and one of
the best ideas is that it's okay to use a subset of the language as long as you get your point across. Any
level of language proficiency is acceptable in Perl culture. We won't send the language police after you.
A Perl script is "correct" if it gets the job done before your boss fires you.
Though simple in many ways, Perl is also a rich language, and there is much to be learned about it. That's
the price of making hard things possible. Although it will take some time for you to absorb all that Perl
can do, you will be glad that you have access to the extensive capabilities of Perl when the time comes
that you need them. We noted above that Perl borrows many capabilities from the shells and C, but Perl
also possesses a strict superset of sed and awk capabilities. There are, in fact, translators supplied with
Perl to turn your old sed and awk scripts into Perl scripts, so you can see how the features you may
already be familiar with correspond to those of Perl.
Because of that heritage, Perl was a rich language even when it was "just" a data-reduction language,
designed for navigating files, scanning large amounts of text, creating and obtaining dynamic data, and
printing easily formatted reports based on that data. But somewhere along the line, Perl started to
blossom. It also became a language for filesystem manipulation, process management, database
administration, client-server programming, secure programming, Web-based information management,
and even for object-oriented and functional programming. These capabilities were not just slapped onto
the side of Perl - each new capability works synergistically with the others, because Perl was designed to
be a glue language from the start.
But Perl can glue together more than its own features. Perl is designed to be modularly extensible. Perl
allows you to rapidly design, program, debug, and deploy applications, but it also allows you to easily
extend the functionality of these applications as the need arises. You can embed Perl in other languages,
and you can embed other languages in Perl. Through the module importation mechanism, you can use
these external definitions as if they were built-in features of Perl. Object-oriented external libraries retain
their object-orientedness in Perl.
Perl helps you in other ways too. Unlike a strictly interpreted language such as the shell, which compiles
and executes a script one command at a time, Perl first compiles your whole program quickly into an
intermediate format. Like any other compiler, it performs various optimizations, and gives you instant
feedback on everything from syntax and semantic errors to library binding mishaps. Once Perl's compiler
frontend is happy with your program, it passes off the intermediate code to the interpreter to execute (or
optionally to any of several modular back ends that can emit C or bytecode.) This all sounds
complicated, but the compiler and interpreter are quite efficient, and most of us find that the typical
compile-run-fix cycle is measured in mere seconds. Together with Perl's many fail-soft characteristics,
this quick turnaround capability makes Perl a language in which you really can do rapid prototyping.
Then later, as your program matures, you can tighten the screws on yourself, and make yourself program
with less flair but more discipline. Perl helps you with that too, if you ask nicely.

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