perlintro.pod

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=head1 NAME

perlintro -- a brief introduction and overview of Perl

=head1 DESCRIPTION

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
I<strongly> advised to follow this introduction with more information
from the full Perl manual, the table of contents to which can be found
in L<perltoc>.

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

=head2 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 L<perl>, L<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.

=head2 Running Perl programs

To run a Perl program from the Unix command line:

    perl progname.pl

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

    #!/usr/bin/env perl

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

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

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

=head2 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:

    #!/usr/bin/perl
    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 C<use strict;> will cause your code to stop
immediately when it is encountered, while C<use warnings;> will merely
give a warning (like the command-line switch B<-w>) and let your code run.
To read more about them check their respective manual pages at L<strict>
and L<warnings>.

=head2 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 C<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:

    print
        "Hello, world"
        ;

... except inside quoted strings:

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

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 (C<\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 L<perlsyn>.

=head2 Perl variable types

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

=over 4

=item Scalars

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 C<my> keyword the first time you use them. (This is one of the
requirements of C<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 L<perlvar>.  The only one you
need to know about for now is C<$_> 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 constructs.

    print;          # prints contents of $_ by default

=item Arrays

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 array:

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

The special variable C<$#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 C<$#array + 1> to tell you how many items there
are in an array.  Don't bother.  As it happens, using C<@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 C<$> 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 C<@ARGV> (the command
line arguments to your script) and C<@_> (the arguments passed to a
subroutine).  These are documented in L<perlvar>.

=item Hashes

A hash represents a set of key/value pairs:

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

You can use whitespace and the C<< => >> operator to lay them out more
nicely:

    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 C<keys()> and
C<values()>.

    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 C<%ENV> which contains environment
variables.  Read all about it (and other special variables) in
L<perlvar>.

=back

Scalars, arrays and hashes are documented more fully in L<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
L<perlreftut>, L<perllol>, L<perlref> and L<perldsc>.

=head2 Variable scoping

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

    my $var = "value";

The C<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.  C<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 defined.

    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 C<my> in combination with a C<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
C<print $b> would cause a compile-time error and prevent you from
running the program.  Using C<strict> is highly recommended.

=head2 Conditional and looping constructs

Perl has most of the usual conditional and looping constructs except for
case/switch (but if you really want it, there is a Switch module in Perl
5.8 and newer, and on CPAN. See the section on modules, below, for more
information about modules and CPAN).

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.

=over 4

=item if

    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 C<if (!I<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