---
name: perl
category: language
language: perl
filename: learnperl.pl
contributors:
    - ["Korjavin Ivan", "http://github.com/korjavin"]
    - ["Dan Book", "http://github.com/Grinnz"]
---

Perl is a highly capable, feature-rich programming language with over 25 years of development.

Perl runs on over 100 platforms from portables to mainframes and is suitable for both rapid prototyping and large scale development projects.

```perl
# Single line comments start with a number sign.

#### Strict and warnings

use strict;
use warnings;

# All perl scripts and modules should include these lines. Strict causes
# compilation to fail in cases like misspelled variable names, and
# warnings will print warning messages in case of common pitfalls like
# concatenating to an undefined value.

#### Perl variable types

#  Variables begin with a sigil, which is a symbol showing the type.
#  A valid variable name starts with a letter or underscore,
#  followed by any number of letters, numbers, or underscores.

### Perl has three main variable types: $scalar, @array, and %hash.

## Scalars
#  A scalar represents a single value:
my $animal = "camel";
my $answer = 42;
my $display = "You have $answer ${animal}s.\n";

# Scalar values can be strings, integers or floating point numbers, and
# Perl will automatically convert between them as required.

# Strings in single quotes are literal strings. Strings in double quotes
# will interpolate variables and escape codes like "\n" for newline.

## Arrays
#  An array represents a list of values:
my @animals = ("camel", "llama", "owl");
my @numbers = (23, 42, 69);
my @mixed   = ("camel", 42, 1.23);

# Array elements are accessed using square brackets, with a $ to
# indicate one value will be returned.
my $second = $animals[1];

# The size of an array is retrieved by accessing the array in a scalar
# context, such as assigning it to a scalar variable or using the
# "scalar" operator.

my $num_animals = @animals;
print "Number of numbers: ", scalar(@numbers), "\n";

# Arrays can also be interpolated into double-quoted strings, and the
# elements are separated by a space character by default.

print "We have these numbers: @numbers\n";

# Be careful when using double quotes for strings containing symbols
# such as email addresses, as it will be interpreted as a variable.

my @example = ('secret', 'array');
my $oops_email = "foo@example.com"; # 'foosecret array.com'
my $ok_email = 'foo@example.com';

## Hashes
#   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
#  nicely:

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

# Hash elements are accessed using curly braces, again with the $ sigil.
my $color = $fruit_color{apple};

# All of the keys or values that exist in a hash can be accessed using
# the "keys" and "values" functions.
my @fruits = keys %fruit_color;
my @colors = values %fruit_color;

# Scalars, arrays and hashes are documented more fully in perldata.
# (perldoc perldata).

#### References

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

my $array_ref = \@array;
my $hash_ref = \%hash;
my @array_of_arrays = (\@array1, \@array2, \@array3);

# You can also create anonymous arrays or hashes, returning a reference:

my $fruits = ["apple", "banana"];
my $colors = {apple => "red", banana => "yellow"};

# References can be dereferenced by prefixing the appropriate sigil.

my @fruits_array = @$fruits;
my %colors_hash = %$colors;

# As a shortcut, the arrow operator can be used to dereference and
# access a single value.

my $first = $array_ref->[0];
my $value = $hash_ref->{banana};

# See perlreftut and perlref for more in-depth documentation on
# references.

#### Conditional and looping constructs

# Perl has most of the usual conditional and looping constructs.

if ($var) {
  ...
} elsif ($var eq 'bar') {
  ...
} else {
  ...
}

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

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

#  while
while (condition) {
  ...
}

my $max = 5;
# for loops and iteration
for my $i (0 .. $max) {
  print "index is $i";
}

for my $element (@elements) {
  print $element;
}

map {print} @elements;

# implicitly

for (@elements) {
  print;
}

# iterating through a hash (for and foreach are equivalent)

foreach my $key (keys %hash) {
  print $key, ': ', $hash{$key}, "\n";
}

# the Perlish post-condition way again
print for @elements;

# iterating through the keys and values of a referenced hash
print $hash_ref->{$_} for keys %$hash_ref;

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

# Simple substitution

$x =~ s/foo/bar/;         # replaces foo with bar in $x
$x =~ s/foo/bar/g;        # replaces ALL INSTANCES of foo with bar in $x


#### Files and I/O

# You can open a file for input or output using the "open()" function.

# For reading:
open(my $in,  "<",  "input.txt")  or die "Can't open input.txt: $!";
# For writing (clears file if it exists):
open(my $out, ">",  "output.txt") or die "Can't open output.txt: $!";
# For writing (appends to end of file):
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>;

# You can iterate through the lines in a file one at a time with a while loop:

while (my $line = <$in>) {
  print "Found apples\n" if $line =~ m/apples/;
}

# You can write to an open filehandle using the standard "print"
# function.

print $out @lines;
print $log $msg, "\n";

#### 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!");

#### Modules

# A module is a set of Perl code, usually subroutines, which can be used
# in other Perl code. It is usually stored in a file with the extension
# .pm so that Perl can find it.

package MyModule;
use strict;
use warnings;

sub trim {
  my $string = shift;
  $string =~ s/^\s+//;
  $string =~ s/\s+$//;
  return $string;
}

1;

# From elsewhere:

use MyModule;
MyModule::trim($string);

# The Exporter module can help with making subroutines exportable, so
# they can be used like this:

use MyModule 'trim';
trim($string);

# Many Perl modules can be downloaded from CPAN (http://www.cpan.org/)
# and provide a range of features to help you avoid reinventing the
# wheel.  A number of popular modules like Exporter are included with
# the Perl distribution itself. See perlmod for more details on modules
# in Perl.

#### Objects

# Objects in Perl are just references that know which class (package)
# they belong to, so that methods (subroutines) called on it can be
# found there. The bless function is used in constructors (usually new)
# to set this up. However, you never need to call it yourself if you use
# a module like Moose or Moo (see below).

package MyCounter;
use strict;
use warnings;

sub new {
  my $class = shift;
  my $self = {count => 0};
  return bless $self, $class;
}

sub count {
  my $self = shift;
  return $self->{count};
}

sub increment {
  my $self = shift;
  $self->{count}++;
}

1;

# Methods can be called on a class or object instance with the arrow
# operator.

use MyCounter;
my $counter = MyCounter->new;
print $counter->count, "\n"; # 0
$counter->increment;
print $counter->count, "\n"; # 1

# The modules Moose and Moo from CPAN can help you set up your object
# classes. They provide a constructor and simple syntax for declaring
# attributes. This class can be used equivalently to the one above.

package MyCounter;
use Moo; # imports strict and warnings

has 'count' => (is => 'rwp', default => 0, init_arg => undef);

sub increment {
  my $self = shift;
  $self->_set_count($self->count + 1);
}

1;

# Object-oriented programming is covered more thoroughly in perlootut,
# and its low-level implementation in Perl is covered in perlobj.
```

#### FAQ

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

#### Further Reading

 - [perl-tutorial](http://perl-tutorial.org/)
 - [Learn at www.perl.com](http://www.perl.org/learn.html)
 - [perldoc](http://perldoc.perl.org/)
 - and perl built-in : `perldoc perlintro`
