perlintro -- a brief introduction and overview of Perl


   This document is intended to give you a quick overview of the Perl
   programming language, along with pointers to further documentation.  It
   is intended as a "bootstrap" guide for those who are new to the
   language, and provides just enough information for you to be able to
   read other peoples' Perl and understand roughly what it's doing, or
   write your own simple scripts.

   This introductory document does not aim to be complete.  It does not
   even aim to be entirely accurate.  In some cases perfection has been
   sacrificed in the goal of getting the general idea across.  You are
   strongly advised to follow this introduction with more information from
   the full Perl manual, the table of contents to which can be found in

   Throughout this document you'll see references to other parts of the
   Perl documentation.  You can read that documentation using the
   "perldoc" command or whatever method you're using to read this

   Throughout Perl's documentation, you'll find numerous examples intended
   to help explain the discussed features.  Please keep in mind that many
   of them are code fragments rather than complete programs.

   These examples often reflect the style and preference of the author of
   that piece of the documentation, and may be briefer than a
   corresponding line of code in a real program.  Except where otherwise
   noted, you should assume that "use strict" and "use warnings"
   statements appear earlier in the "program", and that any variables used
   have already been declared, even if those declarations have been
   omitted to make the example easier to read.

   Do note that the examples have been written by many different authors
   over a period of several decades.  Styles and techniques will therefore
   differ, although some effort has been made to not vary styles too
   widely in the same sections.  Do not consider one style to be better
   than others - "There's More Than One Way To Do It" is one of Perl's
   mottos.  After all, in your journey as a programmer, you are likely to
   encounter different styles.

   What is Perl?
   Perl is a general-purpose programming language originally developed for
   text manipulation and now used for a wide range of tasks including
   system administration, web development, network programming, GUI
   development, and more.

   The language is intended to be practical (easy to use, efficient,
   complete) rather than beautiful (tiny, elegant, minimal).  Its major
   features are that it's easy to use, supports both procedural and
   object-oriented (OO) programming, has powerful built-in support for
   text processing, and has one of the world's most impressive collections
   of third-party modules.

   Different definitions of Perl are given in perl, perlfaq1 and no doubt
   other places.  From this we can determine that Perl is different things
   to different people, but that lots of people think it's at least worth
   writing about.

   Running Perl programs
   To run a Perl program from the Unix command line:


   Alternatively, put this as the first line of your script:

    #!/usr/bin/env perl

   ... and run the script as /path/to/  Of course, it'll need to
   be executable first, so "chmod 755" (under Unix).

   (This start line assumes you have the env program.  You can also put
   directly the path to your perl executable, like in "#!/usr/bin/perl").

   For more information, including instructions for other platforms such
   as Windows and Mac OS, read perlrun.

   Safety net
   Perl by default is very forgiving.  In order to make it more robust it
   is recommended to start every program with the following lines:

    use strict;
    use warnings;

   The two additional lines request from perl to catch various common
   problems in your code.  They check different things so you need both.
   A potential problem caught by "use strict;" will cause your code to
   stop immediately when it is encountered, while "use warnings;" will
   merely give a warning (like the command-line switch -w) and let your
   code run.  To read more about them check their respective manual pages
   at strict and warnings.

   Basic syntax overview
   A Perl script or program consists of one or more statements.  These
   statements are simply written in the script in a straightforward
   fashion.  There is no need to have a "main()" function or anything of
   that kind.

   Perl statements end in a semi-colon:

    print "Hello, world";

   Comments start with a hash symbol and run to the end of the line

    # This is a comment

   Whitespace is irrelevant:

        "Hello, world"

   ... except inside quoted strings:

    # this would print with a linebreak in the middle
    print "Hello

   Double quotes or single quotes may be used around literal strings:

    print "Hello, world";
    print 'Hello, world';

   However, only double quotes "interpolate" variables and special
   characters such as newlines ("\n"):

    print "Hello, $name\n";     # works fine
    print 'Hello, $name\n';     # prints $name\n literally

   Numbers don't need quotes around them:

    print 42;

   You can use parentheses for functions' arguments or omit them according
   to your personal taste.  They are only required occasionally to clarify
   issues of precedence.

    print("Hello, world\n");
    print "Hello, world\n";

   More detailed information about Perl syntax can be found in perlsyn.

   Perl variable types
   Perl has three main variable types: scalars, arrays, and hashes.

       A scalar represents a single value:

        my $animal = "camel";
        my $answer = 42;

       Scalar values can be strings, integers or floating point numbers,
       and Perl will automatically convert between them as required.
       There is no need to pre-declare your variable types, but you have
       to declare them using the "my" keyword the first time you use them.
       (This is one of the requirements of "use strict;".)

       Scalar values can be used in various ways:

        print $animal;
        print "The animal is $animal\n";
        print "The square of $answer is ", $answer * $answer, "\n";

       There are a number of "magic" scalars with names that look like
       punctuation or line noise.  These special variables are used for
       all kinds of purposes, and are documented in perlvar.  The only one
       you need to know about for now is $_ which is the "default
       variable".  It's used as the default argument to a number of
       functions in Perl, and it's set implicitly by certain looping

        print;          # prints contents of $_ by default

       An array represents a list of values:

        my @animals = ("camel", "llama", "owl");
        my @numbers = (23, 42, 69);
        my @mixed   = ("camel", 42, 1.23);

       Arrays are zero-indexed.  Here's how you get at elements in an

        print $animals[0];              # prints "camel"
        print $animals[1];              # prints "llama"

       The special variable $#array tells you the index of the last
       element of an array:

        print $mixed[$#mixed];       # last element, prints 1.23

       You might be tempted to use "$#array + 1" to tell you how many
       items there are in an array.  Don't bother.  As it happens, using
       @array where Perl expects to find a scalar value ("in scalar
       context") will give you the number of elements in the array:

        if (@animals < 5) { ... }

       The elements we're getting from the array start with a "$" because
       we're getting just a single value out of the array; you ask for a
       scalar, you get a scalar.

       To get multiple values from an array:

        @animals[0,1];                 # gives ("camel", "llama");
        @animals[0..2];                # gives ("camel", "llama", "owl");
        @animals[1..$#animals];        # gives all except the first element

       This is called an "array slice".

       You can do various useful things to lists:

        my @sorted    = sort @animals;
        my @backwards = reverse @numbers;

       There are a couple of special arrays too, such as @ARGV (the
       command line arguments to your script) and @_ (the arguments passed
       to a subroutine).  These are documented in perlvar.

       A hash represents a set of key/value pairs:

        my %fruit_color = ("apple", "red", "banana", "yellow");

       You can use whitespace and the "=>" operator to lay them out more

        my %fruit_color = (
            apple  => "red",
            banana => "yellow",

       To get at hash elements:

        $fruit_color{"apple"};           # gives "red"

       You can get at lists of keys and values with "keys()" and

        my @fruits = keys %fruit_colors;
        my @colors = values %fruit_colors;

       Hashes have no particular internal order, though you can sort the
       keys and loop through them.

       Just like special scalars and arrays, there are also special
       hashes.  The most well known of these is %ENV which contains
       environment variables.  Read all about it (and other special
       variables) in perlvar.

   Scalars, arrays and hashes are documented more fully in perldata.

   More complex data types can be constructed using references, which
   allow you to build lists and hashes within lists and hashes.

   A reference is a scalar value and can refer to any other Perl data
   type.  So by storing a reference as the value of an array or hash
   element, you can easily create lists and hashes within lists and
   hashes.  The following example shows a 2 level hash of hash structure
   using anonymous hash references.

    my $variables = {
        scalar  =>  {
                     description => "single item",
                     sigil => '$',
        array   =>  {
                     description => "ordered list of items",
                     sigil => '@',
        hash    =>  {
                     description => "key/value pairs",
                     sigil => '%',

    print "Scalars begin with a $variables->{'scalar'}->{'sigil'}\n";

   Exhaustive information on the topic of references can be found in
   perlreftut, perllol, perlref and perldsc.

   Variable scoping
   Throughout the previous section all the examples have used the syntax:

    my $var = "value";

   The "my" is actually not required; you could just use:

    $var = "value";

   However, the above usage will create global variables throughout your
   program, which is bad programming practice.  "my" creates lexically
   scoped variables instead.  The variables are scoped to the block (i.e.
   a bunch of statements surrounded by curly-braces) in which they are

    my $x = "foo";
    my $some_condition = 1;
    if ($some_condition) {
        my $y = "bar";
        print $x;           # prints "foo"
        print $y;           # prints "bar"
    print $x;               # prints "foo"
    print $y;               # prints nothing; $y has fallen out of scope

   Using "my" in combination with a "use strict;" at the top of your Perl
   scripts means that the interpreter will pick up certain common
   programming errors.  For instance, in the example above, the final
   "print $y" would cause a compile-time error and prevent you from
   running the program.  Using "strict" is highly recommended.

   Conditional and looping constructs
   Perl has most of the usual conditional and looping constructs.  As of
   Perl 5.10, it even has a case/switch statement (spelled
   "given"/"when").  See "Switch Statements" in perlsyn for more details.

   The conditions can be any Perl expression.  See the list of operators
   in the next section for information on comparison and boolean logic
   operators, which are commonly used in conditional statements.

        if ( condition ) {
        } elsif ( other condition ) {
        } else {

       There's also a negated version of it:

        unless ( condition ) {

       This is provided as a more readable version of "if (!condition)".

       Note that the braces are required in Perl, even if you've only got
       one line in the block.  However, there is a clever way of making
       your one-line conditional blocks more English like:

        # the traditional way
        if ($zippy) {
            print "Yow!";

        # the Perlish post-condition way
        print "Yow!" if $zippy;
        print "We have no bananas" unless $bananas;

        while ( condition ) {

       There's also a negated version, for the same reason we have

        until ( condition ) {

       You can also use "while" in a post-condition:

        print "LA LA LA\n" while 1;          # loops forever

   for Exactly like C:

        for ($i = 0; $i <= $max; $i++) {

       The C style for loop is rarely needed in Perl since Perl provides
       the more friendly list scanning "foreach" loop.

        foreach (@array) {
            print "This element is $_\n";

        print $list[$_] foreach 0 .. $max;

        # you don't have to use the default $_ either...
        foreach my $key (keys %hash) {
            print "The value of $key is $hash{$key}\n";

       The "foreach" keyword is actually a synonym for the "for" keyword.
       See ""Foreach Loops" in perlsyn".

   For more detail on looping constructs (and some that weren't mentioned
   in this overview) see perlsyn.

   Builtin operators and functions
   Perl comes with a wide selection of builtin functions.  Some of the
   ones we've already seen include "print", "sort" and "reverse".  A list
   of them is given at the start of perlfunc and you can easily read about
   any given function by using "perldoc -f functionname".

   Perl operators are documented in full in perlop, but here are a few of
   the most common ones:

        +   addition
        -   subtraction
        *   multiplication
        /   division

   Numeric comparison
        ==  equality
        !=  inequality
        <   less than
        >   greater than
        <=  less than or equal
        >=  greater than or equal

   String comparison
        eq  equality
        ne  inequality
        lt  less than
        gt  greater than
        le  less than or equal
        ge  greater than or equal

       (Why do we have separate numeric and string comparisons?  Because
       we don't have special variable types, and Perl needs to know
       whether to sort numerically (where 99 is less than 100) or
       alphabetically (where 100 comes before 99).

   Boolean logic
        &&  and
        ||  or
        !   not

       ("and", "or" and "not" aren't just in the above table as
       descriptions of the operators.  They're also supported as operators
       in their own right.  They're more readable than the C-style
       operators, but have different precedence to "&&" and friends.
       Check perlop for more detail.)

        =   assignment
        .   string concatenation
        x   string multiplication
        ..  range operator (creates a list of numbers or strings)

   Many operators can be combined with a "=" as follows:

    $a += 1;        # same as $a = $a + 1
    $a -= 1;        # same as $a = $a - 1
    $a .= "\n";     # same as $a = $a . "\n";

   Files and I/O
   You can open a file for input or output using the "open()" function.
   It's documented in extravagant detail in perlfunc and perlopentut, but
   in short:

    open(my $in,  "<",  "input.txt")  or die "Can't open input.txt: $!";
    open(my $out, ">",  "output.txt") or die "Can't open output.txt: $!";
    open(my $log, ">>", "my.log")     or die "Can't open my.log: $!";

   You can read from an open filehandle using the "<>" operator.  In
   scalar context it reads a single line from the filehandle, and in list
   context it reads the whole file in, assigning each line to an element
   of the list:

    my $line  = <$in>;
    my @lines = <$in>;

   Reading in the whole file at one time is called slurping.  It can be
   useful but it may be a memory hog.  Most text file processing can be
   done a line at a time with Perl's looping constructs.

   The "<>" operator is most often seen in a "while" loop:

    while (<$in>) {     # assigns each line in turn to $_
        print "Just read in this line: $_";

   We've already seen how to print to standard output using "print()".
   However, "print()" can also take an optional first argument specifying
   which filehandle to print to:

    print STDERR "This is your final warning.\n";
    print $out $record;
    print $log $logmessage;

   When you're done with your filehandles, you should "close()" them
   (though to be honest, Perl will clean up after you if you forget):

    close $in or die "$in: $!";

   Regular expressions
   Perl's regular expression support is both broad and deep, and is the
   subject of lengthy documentation in perlrequick, perlretut, and
   elsewhere.  However, in short:

   Simple matching
        if (/foo/)       { ... }  # true if $_ contains "foo"
        if ($a =~ /foo/) { ... }  # true if $a contains "foo"

       The "//" matching operator is documented in perlop.  It operates on
       $_ by default, or can be bound to another variable using the "=~"
       binding operator (also documented in perlop).

   Simple substitution
        s/foo/bar/;               # replaces foo with bar in $_
        $a =~ s/foo/bar/;         # replaces foo with bar in $a
        $a =~ s/foo/bar/g;        # replaces ALL INSTANCES of foo with bar
                                  # in $a

       The "s///" substitution operator is documented in perlop.

   More complex regular expressions
       You don't just have to match on fixed strings.  In fact, you can
       match on just about anything you could dream of by using more
       complex regular expressions.  These are documented at great length
       in perlre, but for the meantime, here's a quick cheat sheet:

        .                   a single character
        \s                  a whitespace character (space, tab, newline,
        \S                  non-whitespace character
        \d                  a digit (0-9)
        \D                  a non-digit
        \w                  a word character (a-z, A-Z, 0-9, _)
        \W                  a non-word character
        [aeiou]             matches a single character in the given set
        [^aeiou]            matches a single character outside the given
        (foo|bar|baz)       matches any of the alternatives specified

        ^                   start of string
        $                   end of string

       Quantifiers can be used to specify how many of the previous thing
       you want to match on, where "thing" means either a literal
       character, one of the metacharacters listed above, or a group of
       characters or metacharacters in parentheses.

        *                   zero or more of the previous thing
        +                   one or more of the previous thing
        ?                   zero or one of the previous thing
        {3}                 matches exactly 3 of the previous thing
        {3,6}               matches between 3 and 6 of the previous thing
        {3,}                matches 3 or more of the previous thing

       Some brief examples:

        /^\d+/              string starts with one or more digits
        /^$/                nothing in the string (start and end are
        /(\d\s){3}/         three digits, each followed by a whitespace
                            character (eg "3 4 5 ")
        /(a.)+/             matches a string in which every odd-numbered
                            letter is a (eg "abacadaf")

        # This loop reads from STDIN, and prints non-blank lines:
        while (<>) {
            next if /^$/;

   Parentheses for capturing
       As well as grouping, parentheses serve a second purpose.  They can
       be used to capture the results of parts of the regexp match for
       later use.  The results end up in $1, $2 and so on.

        # a cheap and nasty way to break an email address up into parts

        if ($email =~ /([^@]+)@(.+)/) {
            print "Username is $1\n";
            print "Hostname is $2\n";

   Other regexp features
       Perl regexps also support backreferences, lookaheads, and all kinds
       of other complex details.  Read all about them in perlrequick,
       perlretut, and perlre.

   Writing subroutines
   Writing subroutines is easy:

    sub logger {
       my $logmessage = shift;
       open my $logfile, ">>", "my.log" or die "Could not open my.log: $!";
       print $logfile $logmessage;

   Now we can use the subroutine just as any other built-in function:

    logger("We have a logger subroutine!");

   What's that "shift"?  Well, the arguments to a subroutine are available
   to us as a special array called @_ (see perlvar for more on that).  The
   default argument to the "shift" function just happens to be @_.  So "my
   $logmessage = shift;" shifts the first item off the list of arguments
   and assigns it to $logmessage.

   We can manipulate @_ in other ways too:

    my ($logmessage, $priority) = @_;       # common
    my $logmessage = $_[0];                 # uncommon, and ugly

   Subroutines can also return values:

    sub square {
        my $num = shift;
        my $result = $num * $num;
        return $result;

   Then use it like:

    $sq = square(8);

   For more information on writing subroutines, see perlsub.

   OO Perl
   OO Perl is relatively simple and is implemented using references which
   know what sort of object they are based on Perl's concept of packages.
   However, OO Perl is largely beyond the scope of this document.  Read
   perlootut and perlobj.

   As a beginning Perl programmer, your most common use of OO Perl will be
   in using third-party modules, which are documented below.

   Using Perl modules
   Perl modules provide a range of features to help you avoid reinventing
   the wheel, and can be downloaded from CPAN ( ).  A
   number of popular modules are included with the Perl distribution

   Categories of modules range from text manipulation to network protocols
   to database integration to graphics.  A categorized list of modules is
   also available from CPAN.

   To learn how to install modules you download from CPAN, read

   To learn how to use a particular module, use "perldoc Module::Name".
   Typically you will want to "use Module::Name", which will then give you
   access to exported functions or an OO interface to the module.

   perlfaq contains questions and answers related to many common tasks,
   and often provides suggestions for good CPAN modules to use.

   perlmod describes Perl modules in general.  perlmodlib lists the
   modules which came with your Perl installation.

   If you feel the urge to write Perl modules, perlnewmod will give you
   good advice.


   Kirrily "Skud" Robert <>


Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.

Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.

Free Software

Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.

Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.

Free Books

The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.

Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.


Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.

Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.