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expected in first half of 2021 and release candidates sooner 83 this plan was revised in may 2021 without any release timeframe or version of perl 5 for use as a baseline specified 84 when perl 7 would be released perl 5 would have gone into long term maintenance supported perl 5 versions however would continue to get important security and bug fixes 85 perl 7 was announced on 24 june 2020 at the perl conference in the cloud as the successor to perl 5 83 82 based on perl 5 32 perl 7 was planned to be backward compatible with modern perl 5 code perl 5 code without boilerplate pragma header needs adding use compat perl5 to stay compatible but modern code can drop some of the boilerplate the plan to go to perl 7 brought up more discussion however and the perl steering committee canceled it to avoid issues with backward compatibility for scripts that were not written to the pragmas and modules that would become the default in perl 7 perl 7 will only come out when the developers add enough features to warrant a major release upgrade 86 design edit main article perl language structure philosophy edit according to wall perl has two slogans the first is there s more than one way to do it commonly known as tmtowtdi pronounced tim toady proponents of this motto argue that this philosophy makes it easy to write concise statements 87 88 89 the second slogan is easy things should be easy and hard things should be possible 14 the design of perl can be understood as a response to three broad trends in the computer industry falling hardware costs rising labor costs and improvements in compiler technology many earlier computer languages such as fortran and c aimed to make efficient use of expensive computer hardware in contrast perl was designed so that computer programmers could write programs more quickly and easily 90 perl has many features that ease the task of the programmer at the expense of greater cpu and memory requirements these include automatic memory management dynamic typing strings lists and hashes regular expressions introspection and an eval function perl follows the theory of no built in limits 27 an idea similar to the zero one infinity rule wall was trained as a linguist and the design of perl is very much informed by linguistic principles examples include huffman coding common constructions should be short good end weighting the important information should come first and a large collection of language primitives perl favors language constructs that are concise and natural for humans to write even where they complicate the perl interpreter 91 perl s syntax reflects the idea that things that are different should look different 92 for example scalars arrays and hashes have different leading sigils array indices and hash keys use different kinds of braces strings and regular expressions have different standard delimiters there is a broad practical bent to both the perl language and the community and culture that surround it the preface to programming perl begins perl is a language for getting your job done 14 one consequence of this is that perl is not a tidy language it includes many features tolerates exceptions to its rules and employs heuristics to resolve syntactical ambiguities because of the forgiving nature of the compiler bugs can sometimes be hard to find perl s function documentation remarks on the variant behavior of built in functions in list and scalar contexts by saying in general they do what you want unless you want consistency 93 features edit the overall structure of perl derives broadly from c perl is procedural in nature with variables expressions assignment statements brace delimited blocks control structures and subroutines 94 perl also takes features from shell programming all variables are marked with leading sigils which allow variables to be interpolated directly into strings however unlike the shell perl uses sigils on all accesses to variables and unlike most other programming languages that use sigils the sigil doesn t denote the type of the variable but the type of the expression so for example while an array is denoted by the sigil for example arrayname an individual member of the array is denoted by the scalar sigil for example arrayname 3 perl also has many built in functions that provide tools often used in shell programming although many of these tools are implemented by programs external to the shell such as sorting and calling operating system facilities citation needed perl takes hashes associative arrays from awk and regular expressions from sed these simplify many parsing text handling and data management tasks shared with lisp is the implicit return of the last value in a block and all statements are also expressions which can be used in larger expressions themselves citation needed perl 5 added features that support complex data structures first class functions that is closures as values and an object oriented programming model these include references packages class based method dispatch and lexically scoped variables along with compiler directives for example the strict pragma a major additional feature introduced with perl 5 was the ability to package code as reusable modules wall later stated that the whole intent of perl 5 s module system was to encourage the growth of perl culture rather than the perl core 95 all versions of perl do automatic data typing and automatic memory management the interpreter knows the type and storage requirements of every data object in the program it allocates and frees storage for them as necessary using reference counting so it cannot deallocate circular data structures without manual intervention legal type conversions for example conversions from number to string are done automatically at run time illegal type conversions are fatal errors citation needed syntax edit perl has been referred to as line noise and a write only language by its critics randal l schwartz in the first edition of the book learning perl 96 in the first chapter states yes sometimes perl looks like line noise to the uninitiated but to the seasoned perl programmer it looks like checksummed line noise with a mission in life 97 he also stated that the accusation that perl is a write only language could be avoided by coding with proper care 97 the perl overview document perlintro states that the names of built in magic scalar variables look like punctuation or line noise 98 however the english module provides both long and short english alternatives perlstyle document states that line noise in regular expressions could be mitigated using the x modifier to add whitespace 99 according to the perl 6 faq 100 perl 6 was designed to mitigate the usual suspects that elicit the line noise claim from perl 5 critics including the removal of the majority of the punctuation variables and the sanitization of the regex syntax the perl 6 faq also states that what is sometimes referred to as perl s line noise is the actual syntax of the language just as gerunds and prepositions are a part of the english language 100 in a december 2012 blog posting despite claiming that rakudo perl 6 has failed and will continue to fail unless it gets some adult supervision chromatic stated that the design of perl 6 has a well defined grammar an improved type system a unified object system with an intelligent metamodel metaoperators and a clearer system of context that provides for such niceties as pervasive laziness 101 he also stated that perl 6 has a coherence and a consistency that perl 5 lacks 101 in perl one could write the hello world program as print hello world n here is a more complex perl program that counts down seconds from a given starting value usr bin env perl use strict use warnings my remaining total remaining total shift argv stdout autoflush 1 while remaining printf remaining s s r remaining total sleep 1 print n to run the code above store it in a file named counter pl and then execute it perl counter pl 42 the perl interpreter can also be used for one off scripts on the command line the following example as invoked from an sh compatible shell such as bash translates the string bob in all files ending with txt in the current directory to robert perl i bak lp e s bob robert g txt implementation edit no written specification or standard for the perl language exists for perl versions through perl 5 and there are no plans to create one for the current version of perl there has been only one implementation of the interpreter and the language has evolved along with it that interpreter together with its functional tests stands as a de facto specification of the language perl 6 however started with a specification 102 and several projects 103 aim to implement some or all of the specification citation needed perl is implemented as a core interpreter written in c together with a large collection of modules written in perl and c as of 2010 update the interpreter is 150 000 lines of c code and compiles to a 1 mb executable on typical machine architectures alternatively the interpreter can be compiled to a link library and embedded in other programs there are nearly 500 modules in the distribution comprising 200 000 lines of perl and an additional 350 000 lines of c code much of the c code in the modules consists of character encoding tables citation needed the interpreter has an object oriented architecture all of the elements of the perl language scalars arrays hashes coderefs file handles are represented in the interpreter by c structs operations on these structs are defined by a large collection of macros typedefs and functions these constitute the perl c api the perl api can be bewildering to the uninitiated but its entry points follow a consistent naming scheme which provides guidance to those who use it citation needed the life of a perl interpreter divides broadly into a compile phase and a run phase 104 according to aluín et al perl cannot be parsed by a straight lex yacc lexer parser combination instead the interpreter implements its own lexer which coordinates with a modified gnu bison parser to resolve ambiguities in the language 105 most of what happens in perl s compile phase is compilation and most of what happens in perl s run phase is execution but there are significant exceptions perl makes important use of its capability to execute perl code during the compile phase perl will also delay compilation into the run phase the terms that indicate the kind of processing that is actually occurring at any moment are compile time and run time perl is in compile time at most points during the compile phase but compile time may also be entered during the run phase the compile time for code in a string argument passed to the eval built in occurs during the run phase perl is often in run time during the compile phase and spends most of the run phase in run time code in begin blocks executes at run time but in the compile phase at compile time the interpreter parses perl code into a syntax tree at run time it executes the program by walking the tree text is parsed only once and the syntax tree is subject to optimization before it is executed so that execution is relatively efficient compile time optimizations on the syntax tree include constant folding and context propagation but peephole optimization is also performed 106 perl has a turing complete grammar because parsing can be affected by run time code executed during the compile phase 107 the code cannot be parsed by a straight lex yacc lexer parser to resolve ambiguities in the language the interpreter must implement its own lexer to coordinate with a modified gnu bison parser 108 it is often said that only perl can parse perl 109 meaning that only the perl interpreter perl can parse the perl language perl but even this is not in general true because the perl interpreter can simulate a turing machine during its compile phase it would need to decide the halting problem in order to complete parsing in every case it is a longstanding result that the halting problem is undecidable and therefore not even perl can always parse perl perl makes the unusual choice of giving the user access to its full programming power in its own compile phase the cost in terms of theoretical purity is high but practical inconvenience seems to be rare 110 other programs that undertake to parse perl such as source code analyzers and auto indenters have to contend not only with ambiguous syntactic constructs but also with the undecidability of perl parsing in the general case adam kennedy s ppi project focused on parsing perl code as a document retaining its integrity as a document instead of parsing perl as executable code that not even perl itself can always do it was kennedy who first conjectured that parsing perl suffers from the halting problem 111 which was later proved 112 perl is distributed with over 250 000 functional tests for core perl language and over 250 000 functional tests for core modules these run as part of the normal build process and extensively exercise the interpreter and its core modules perl developers rely on the functional tests to ensure that changes to the interpreter do not introduce software bugs further perl users who see that the interpreter passes its functional tests on their system can have a high degree of confidence that it is working properly citation needed ports edit perl is dual licensed under both the artistic license 1 0 4 5 and the gnu general public license 6 distributions are available for most operating systems it is particularly prevalent on unix and unix like systems but it has been ported to most modern and many obsolete platforms with only six citation needed reported exceptions perl can be compiled from source code on all posix compliant or otherwise unix compatible platforms 113 because of unusual changes required for the classic mac os environment a special port called macperl was shipped independently 114 the comprehensive perl archive network carries a complete list of supported platforms with links to the distributions available on each 115 cpan is also the source for publicly available perl modules that are not part of the core perl distribution citation needed activeperl is a closed source distribution from activestate that has regular releases that track the core perl releases 116 the distribution previously included the perl package manager ppm 117 a popular tool for installing removing upgrading and managing the use of common perl modules however this tool was discontinued as of activeperl 5 28 118 included also is perlscript a windows script host wsh engine implementing the perl language visual perl is an activestate tool that adds perl to the visual studio net development suite a vbscript to perl converter a perl compiler for windows and converters of awk and sed to perl have also been produced by this company and included on the activestate cd for windows 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