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1 | =head1 NAME |
2 | |
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3 | perlfaq1 - General Questions About Perl ($Revision: 1.8 $, $Date: 2002/04/07 18:46:13 $) |
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4 | |
5 | =head1 DESCRIPTION |
6 | |
7 | This section of the FAQ answers very general, high-level questions |
8 | about Perl. |
9 | |
10 | =head2 What is Perl? |
11 | |
12 | Perl is a high-level programming language with an eclectic heritage |
13 | written by Larry Wall and a cast of thousands. It derives from the |
14 | ubiquitous C programming language and to a lesser extent from sed, |
15 | awk, the Unix shell, and at least a dozen other tools and languages. |
16 | Perl's process, file, and text manipulation facilities make it |
17 | particularly well-suited for tasks involving quick prototyping, system |
18 | utilities, software tools, system management tasks, database access, |
19 | graphical programming, networking, and world wide web programming. |
20 | These strengths make it especially popular with system administrators |
21 | and CGI script authors, but mathematicians, geneticists, journalists, |
22 | and even managers also use Perl. Maybe you should, too. |
23 | |
24 | =head2 Who supports Perl? Who develops it? Why is it free? |
25 | |
26 | The original culture of the pre-populist Internet and the deeply-held |
27 | beliefs of Perl's author, Larry Wall, gave rise to the free and open |
28 | distribution policy of perl. Perl is supported by its users. The |
29 | core, the standard Perl library, the optional modules, and the |
30 | documentation you're reading now were all written by volunteers. See |
31 | the personal note at the end of the README file in the perl source |
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32 | distribution for more details. See L<perlhist> (new as of 5.005) |
33 | for Perl's milestone releases. |
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34 | |
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35 | In particular, the core development team (known as the Perl Porters) |
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36 | are a rag-tag band of highly altruistic individuals committed to |
37 | producing better software for free than you could hope to purchase for |
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38 | money. You may snoop on pending developments via the archives at |
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39 | http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/ |
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40 | and http://archive.develooper.com/perl5-porters@perl.org/ |
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41 | or the news gateway nntp://nntp.perl.org/perl.perl5.porters or |
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42 | its web interface at http://nntp.perl.org/group/perl.perl5.porters , |
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43 | or read the faq at http://perlhacker.org/p5p-faq , |
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44 | or you can subscribe to the mailing list by sending |
45 | perl5-porters-request@perl.org a subscription request |
46 | (an empty message with no subject is fine). |
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47 | |
48 | While the GNU project includes Perl in its distributions, there's no |
49 | such thing as "GNU Perl". Perl is not produced nor maintained by the |
50 | Free Software Foundation. Perl's licensing terms are also more open |
51 | than GNU software's tend to be. |
52 | |
53 | You can get commercial support of Perl if you wish, although for most |
54 | users the informal support will more than suffice. See the answer to |
55 | "Where can I buy a commercial version of perl?" for more information. |
56 | |
57 | =head2 Which version of Perl should I use? |
58 | |
59 | You should definitely use version 5. Version 4 is old, limited, and |
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60 | no longer maintained; its last patch (4.036) was in 1992, long ago and |
61 | far away. Sure, it's stable, but so is anything that's dead; in fact, |
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62 | perl4 had been called a dead, flea-bitten camel carcass. The most |
63 | recent production release is 5.8.0 (although 5.005_03 and 5.6.1 are |
64 | still supported). The most cutting-edge development release is 5.9. |
65 | Further references to the Perl language in this document refer to the |
66 | production release unless otherwise specified. There may be one or |
67 | more official bug fixes by the time you read this, and also perhaps |
68 | some experimental versions on the way to the next release. |
69 | All releases prior to 5.004 were subject to buffer overruns, a grave |
70 | security issue. |
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71 | |
72 | =head2 What are perl4 and perl5? |
73 | |
74 | Perl4 and perl5 are informal names for different versions of the Perl |
75 | programming language. It's easier to say "perl5" than it is to say |
76 | "the 5(.004) release of Perl", but some people have interpreted this |
77 | to mean there's a language called "perl5", which isn't the case. |
78 | Perl5 is merely the popular name for the fifth major release (October 1994), |
79 | while perl4 was the fourth major release (March 1991). There was also a |
80 | perl1 (in January 1988), a perl2 (June 1988), and a perl3 (October 1989). |
81 | |
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82 | The 5.0 release is, essentially, a ground-up rewrite of the original |
83 | perl source code from releases 1 through 4. It has been modularized, |
84 | object-oriented, tweaked, trimmed, and optimized until it almost doesn't |
85 | look like the old code. However, the interface is mostly the same, and |
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86 | compatibility with previous releases is very high. |
87 | See L<perltrap/"Perl4 to Perl5 Traps">. |
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88 | |
89 | To avoid the "what language is perl5?" confusion, some people prefer to |
90 | simply use "perl" to refer to the latest version of perl and avoid using |
91 | "perl5" altogether. It's not really that big a deal, though. |
92 | |
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93 | See L<perlhist> for a history of Perl revisions. |
94 | |
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95 | =head2 What is perl6? |
96 | |
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97 | At The Second O'Reilly Open Source Software Convention, Larry Wall |
98 | announced Perl6 development would begin in earnest. Perl6 was an oft |
99 | used term for Chip Salzenberg's project to rewrite Perl in C++ named |
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100 | Topaz. However, Topaz provided valuable insights to the next version |
101 | of Perl and its implementation, but was ultimately abandoned. |
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102 | |
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103 | If you want to learn more about Perl6, or have a desire to help in |
104 | the crusade to make Perl a better place then peruse the Perl6 developers |
105 | page at http://dev.perl.org/perl6/ and get involved. |
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106 | |
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107 | Perl6 is not scheduled for release yet, and Perl5 will still be supported |
108 | for quite awhile after its release. Do not wait for Perl6 to do whatever |
109 | you need to do. |
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110 | |
111 | "We're really serious about reinventing everything that needs reinventing." |
112 | --Larry Wall |
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113 | |
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114 | =head2 How stable is Perl? |
115 | |
116 | Production releases, which incorporate bug fixes and new functionality, |
117 | are widely tested before release. Since the 5.000 release, we have |
118 | averaged only about one production release per year. |
119 | |
120 | Larry and the Perl development team occasionally make changes to the |
121 | internal core of the language, but all possible efforts are made toward |
122 | backward compatibility. While not quite all perl4 scripts run flawlessly |
123 | under perl5, an update to perl should nearly never invalidate a program |
124 | written for an earlier version of perl (barring accidental bug fixes |
125 | and the rare new keyword). |
126 | |
127 | =head2 Is Perl difficult to learn? |
128 | |
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129 | No, Perl is easy to start learning--and easy to keep learning. It looks |
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130 | like most programming languages you're likely to have experience |
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131 | with, so if you've ever written a C program, an awk script, a shell |
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132 | script, or even a BASIC program, you're already partway there. |
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133 | |
134 | Most tasks only require a small subset of the Perl language. One of |
135 | the guiding mottos for Perl development is "there's more than one way |
136 | to do it" (TMTOWTDI, sometimes pronounced "tim toady"). Perl's |
137 | learning curve is therefore shallow (easy to learn) and long (there's |
138 | a whole lot you can do if you really want). |
139 | |
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140 | Finally, because Perl is frequently (but not always, and certainly not by |
141 | definition) an interpreted language, you can write your programs and test |
142 | them without an intermediate compilation step, allowing you to experiment |
143 | and test/debug quickly and easily. This ease of experimentation flattens |
144 | the learning curve even more. |
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145 | |
146 | Things that make Perl easier to learn: Unix experience, almost any kind |
147 | of programming experience, an understanding of regular expressions, and |
148 | the ability to understand other people's code. If there's something you |
149 | need to do, then it's probably already been done, and a working example is |
150 | usually available for free. Don't forget the new perl modules, either. |
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151 | They're discussed in Part 3 of this FAQ, along with CPAN, which is |
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152 | discussed in Part 2. |
153 | |
154 | =head2 How does Perl compare with other languages like Java, Python, REXX, Scheme, or Tcl? |
155 | |
156 | Favorably in some areas, unfavorably in others. Precisely which areas |
157 | are good and bad is often a personal choice, so asking this question |
158 | on Usenet runs a strong risk of starting an unproductive Holy War. |
159 | |
160 | Probably the best thing to do is try to write equivalent code to do a |
161 | set of tasks. These languages have their own newsgroups in which you |
162 | can learn about (but hopefully not argue about) them. |
163 | |
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164 | Some comparison documents can be found at http://language.perl.com/versus/ |
165 | if you really can't stop yourself. |
166 | |
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167 | =head2 Can I do [task] in Perl? |
168 | |
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169 | Perl is flexible and extensible enough for you to use on virtually any |
170 | task, from one-line file-processing tasks to large, elaborate systems. |
171 | For many people, Perl serves as a great replacement for shell scripting. |
172 | For others, it serves as a convenient, high-level replacement for most of |
173 | what they'd program in low-level languages like C or C++. It's ultimately |
174 | up to you (and possibly your management) which tasks you'll use Perl |
175 | for and which you won't. |
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176 | |
177 | If you have a library that provides an API, you can make any component |
178 | of it available as just another Perl function or variable using a Perl |
179 | extension written in C or C++ and dynamically linked into your main |
180 | perl interpreter. You can also go the other direction, and write your |
181 | main program in C or C++, and then link in some Perl code on the fly, |
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182 | to create a powerful application. See L<perlembed>. |
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183 | |
184 | That said, there will always be small, focused, special-purpose |
185 | languages dedicated to a specific problem domain that are simply more |
186 | convenient for certain kinds of problems. Perl tries to be all things |
187 | to all people, but nothing special to anyone. Examples of specialized |
188 | languages that come to mind include prolog and matlab. |
189 | |
190 | =head2 When shouldn't I program in Perl? |
191 | |
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192 | When your manager forbids it--but do consider replacing them :-). |
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193 | |
194 | Actually, one good reason is when you already have an existing |
195 | application written in another language that's all done (and done |
196 | well), or you have an application language specifically designed for a |
197 | certain task (e.g. prolog, make). |
198 | |
199 | For various reasons, Perl is probably not well-suited for real-time |
200 | embedded systems, low-level operating systems development work like |
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201 | device drivers or context-switching code, complex multi-threaded |
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202 | shared-memory applications, or extremely large applications. You'll |
203 | notice that perl is not itself written in Perl. |
204 | |
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205 | The new, native-code compiler for Perl may eventually reduce the |
206 | limitations given in the previous statement to some degree, but understand |
207 | that Perl remains fundamentally a dynamically typed language, not |
208 | a statically typed one. You certainly won't be chastised if you don't |
209 | trust nuclear-plant or brain-surgery monitoring code to it. And Larry |
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210 | will sleep easier, too--Wall Street programs not withstanding. :-) |
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211 | |
212 | =head2 What's the difference between "perl" and "Perl"? |
213 | |
214 | One bit. Oh, you weren't talking ASCII? :-) Larry now uses "Perl" to |
215 | signify the language proper and "perl" the implementation of it, |
216 | i.e. the current interpreter. Hence Tom's quip that "Nothing but perl |
217 | can parse Perl." You may or may not choose to follow this usage. For |
218 | example, parallelism means "awk and perl" and "Python and Perl" look |
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219 | OK, while "awk and Perl" and "Python and perl" do not. But never |
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220 | write "PERL", because perl is not an acronym, apocryphal |
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221 | folklore and post-facto expansions notwithstanding. |
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222 | |
223 | =head2 Is it a Perl program or a Perl script? |
224 | |
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225 | Larry doesn't really care. He says (half in jest) that "a script is |
226 | what you give the actors. A program is what you give the audience." |
227 | |
228 | Originally, a script was a canned sequence of normally interactive |
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229 | commands--that is, a chat script. Something like a UUCP or PPP chat |
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230 | script or an expect script fits the bill nicely, as do configuration |
231 | scripts run by a program at its start up, such F<.cshrc> or F<.ircrc>, |
232 | for example. Chat scripts were just drivers for existing programs, |
233 | not stand-alone programs in their own right. |
234 | |
235 | A computer scientist will correctly explain that all programs are |
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236 | interpreted and that the only question is at what level. But if you |
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237 | ask this question of someone who isn't a computer scientist, they might |
238 | tell you that a I<program> has been compiled to physical machine code |
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239 | once and can then be run multiple times, whereas a I<script> must be |
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240 | translated by a program each time it's used. |
241 | |
242 | Perl programs are (usually) neither strictly compiled nor strictly |
243 | interpreted. They can be compiled to a byte-code form (something of a |
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244 | Perl virtual machine) or to completely different languages, like C or |
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245 | assembly language. You can't tell just by looking at it whether the |
246 | source is destined for a pure interpreter, a parse-tree interpreter, |
247 | a byte-code interpreter, or a native-code compiler, so it's hard to give |
248 | a definitive answer here. |
249 | |
250 | Now that "script" and "scripting" are terms that have been seized by |
251 | unscrupulous or unknowing marketeers for their own nefarious purposes, |
252 | they have begun to take on strange and often pejorative meanings, |
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253 | like "non serious" or "not real programming". Consequently, some Perl |
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254 | programmers prefer to avoid them altogether. |
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255 | |
256 | =head2 What is a JAPH? |
257 | |
258 | These are the "just another perl hacker" signatures that some people |
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259 | sign their postings with. Randal Schwartz made these famous. About |
260 | 100 of the earlier ones are available from |
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261 | http://www.cpan.org/misc/japh . |
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262 | |
263 | =head2 Where can I get a list of Larry Wall witticisms? |
264 | |
265 | Over a hundred quips by Larry, from postings of his or source code, |
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266 | can be found at http://www.cpan.org/misc/lwall-quotes.txt.gz . |
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267 | |
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268 | =head2 How can I convince my sysadmin/supervisor/employees to use version 5/5.6.1/Perl instead of some other language? |
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269 | |
270 | If your manager or employees are wary of unsupported software, or |
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271 | software which doesn't officially ship with your operating system, you |
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272 | might try to appeal to their self-interest. If programmers can be |
273 | more productive using and utilizing Perl constructs, functionality, |
274 | simplicity, and power, then the typical manager/supervisor/employee |
275 | may be persuaded. Regarding using Perl in general, it's also |
276 | sometimes helpful to point out that delivery times may be reduced |
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277 | using Perl compared to other languages. |
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278 | |
279 | If you have a project which has a bottleneck, especially in terms of |
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280 | translation or testing, Perl almost certainly will provide a viable, |
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281 | quick solution. In conjunction with any persuasion effort, you |
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282 | should not fail to point out that Perl is used, quite extensively, and |
283 | with extremely reliable and valuable results, at many large computer |
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284 | software and hardware companies throughout the world. In fact, |
285 | many Unix vendors now ship Perl by default. Support is usually |
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286 | just a news-posting away, if you can't find the answer in the |
287 | I<comprehensive> documentation, including this FAQ. |
288 | |
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289 | See http://www.perl.org/advocacy/ for more information. |
290 | |
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291 | If you face reluctance to upgrading from an older version of perl, |
292 | then point out that version 4 is utterly unmaintained and unsupported |
293 | by the Perl Development Team. Another big sell for Perl5 is the large |
294 | number of modules and extensions which greatly reduce development time |
295 | for any given task. Also mention that the difference between version |
296 | 4 and version 5 of Perl is like the difference between awk and C++. |
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297 | (Well, OK, maybe it's not quite that distinct, but you get the idea.) |
298 | If you want support and a reasonable guarantee that what you're |
299 | developing will continue to work in the future, then you have to run |
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300 | the supported version. As of August 2002 that means running either |
301 | 5.8.0 (released in July 2002), or one of the older releases like |
302 | 5.6.1 (released in April 2001) or 5.005_03 (released in March 1999), |
303 | although 5.004_05 isn't that bad if you B<absolutely> need such an old |
304 | version (released in April 1999) for stability reasons. |
305 | Anything older than 5.004_05 shouldn't be used. |
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306 | |
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307 | Of particular note is the massive bug hunt for buffer overflow |
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308 | problems that went into the 5.004 release. All releases prior to |
309 | that, including perl4, are considered insecure and should be upgraded |
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310 | as soon as possible. |
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311 | |
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312 | In August 2000 in all Linux distributions a new security problem was |
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313 | found in the optional 'suidperl' (not built or installed by default) |
314 | in all the Perl branches 5.6, 5.005, and 5.004, see |
315 | http://www.cpan.org/src/5.0/sperl-2000-08-05/ |
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316 | Perl maintenance releases 5.6.1 and 5.8.0 have this security hole closed. |
317 | Most, if not all, Linux distribution have patches for this |
318 | vulnerability available, see http://www.linuxsecurity.com/advisories/ , |
319 | but the most recommendable way is to upgrade to at least Perl 5.6.1. |
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320 | |
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321 | =head1 AUTHOR AND COPYRIGHT |
322 | |
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323 | Copyright (c) 1997, 1998, 1999, 2000, 2001 Tom Christiansen and Nathan |
324 | Torkington. All rights reserved. |
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325 | |
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326 | This documentation is free; you can redistribute it and/or modify it |
327 | under the same terms as Perl itself. |
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328 | |
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329 | Irrespective of its distribution, all code examples here are in the public |
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330 | domain. You are permitted and encouraged to use this code and any |
331 | derivatives thereof in your own programs for fun or for profit as you |
332 | see fit. A simple comment in the code giving credit to the FAQ would |
333 | be courteous but is not required. |