Encyclopedia of Computer Science and Technology

Perl 365Further ReadingAdobe Creative Team. Adobe Acrobat 8 Classroom in a Book. SanJose, Calif.: Adobe Press, 2007.Adobe Reader (free download). Available online. URL: http://www.adobe.com/products/acrobat/readstep2_allversions.html.Accessed November 4, 2007.Lowagie, Bruno. iText in Action: Creating and Manipulating PDF.New York: Manning Publications, 2007.Padova, Ted. Adobe Acrobat 8 PDF Bible. Indianapolis: Wiley, 2007.Planet PDF: The Home of the PDF Community. Available online.URL: http://www.planetpdf.com/. Accessed November 4,2007.PerlThe explosive growth of the World Wide Web has confrontedprogrammers with the need to find ways to linkdatabases and other existing resources to Web sites. Thespecifications for such linkages are found in the CommonGateway Interface (see CGI). However, the early facilitiesfor writing CGI scripts were awkward and often frustratingto use.Back in 1986, UNIX developer Larry Wall had created alanguage called Perl (Practical Extraction and Report Language).There were already ways to write scripts for simpledata processing (see scripting languages) as well as ahandy pattern-manipulation language (see awk). However,Wall wanted to provide a greater variety of functions andtechniques for finding, extracting, and formatting data. Perlattracted a following within the UNIX community. Sincemuch Web development was being done on UNIX-basedsystems by the mid- and late-1990s, it was natural thatmany webmasters and applications programmers wouldturn to Perl to write their CGI scripts.As with many UNIX scripting languages, Perl’s syntaxis broadly similar to C. However, the philosophy behindC is to provide a sparse core language with most functionalitybeing handled by standard or add-in programlibraries. Perl, on the other hand, starts with most of thefunctionality of UNIX utilities such as sed (stream editor),C shell, and awk, including the powerful regular expressionsfamiliar to UNIX users. The language also includesa “hash” data type (a collection of paired keys and values)that makes it easy for a program to maintain and checklists such as of Internet hosts and their IP addresses (seehashing).Wall made it a point to solicit and respond to feedbackfrom Perl users, often by adding features or functions.Wall’s approach has been to provide as much practical helpfor programmers as possible, rather than worrying aboutthe language being well-defined, consistent, and thus easyto learn. For example, in most languages, to make somethinghappen only if a certain condition is not true, onewrites something like this:If ! (test for valid data)Print Error-Msg;Else Process_Data;In Perl, however, one can use the “unless” clause. Itlooks like this:Unless (Test for invalid data) {Process_Data;}Syntactically, the unless clause does not provide anythingmore than using an If and Else would, and it involves learninga different structure. However, it has the practical benefitof making the program a little easier to read by keeping theemphasis on what the program expects to be doing, not onthe possible error. Similarly, Perl offers an “until” loop:Until (Condition is met) {Do something;}In C, one would have to sayWhile (Condition is not met) {Do something;}This “Swiss army knife” approach to providing languagefeatures has been criticized by some computer scientistsas encouraging undisciplined and hard-to-verify programming.However, Perl’s many aficionados see the languageas the versatile, essential toolbox for the ever-challengingworld of Web programming. As the language evolvedthrough the late 1990s, it also added a full set of object-orientedfeatures (see object-oriented programming).Sample Perl ProgramThe following very simple code illustrates a Perl programthat reads some lines of data from a file and prints themout. The first line tells UNIX to execute the Perl interpreter.The file name data.txt is assigned to the string variable$file. The file is then opened and assigned to the variableINFO. A single statement (not a loop) suffices to assign allthe lines in the file to the array @lines. The “foreach” statementis a compact form of For loop that assigns each linein the array to the string variable $line and then prints it tothe screen as HTML.#!/usr/local/bin/perl$file = ’data.txt’;open(INFO, “