Add info on references, modules, and objects

These topics are central to modern usage of Perl.

perl.html.markdown

@@ -5,6 +5,7 @@ language: perl
 filename: learnperl.pl
 contributors:
     - ["Korjavin Ivan", "http://github.com/korjavin"]
+    - ["Dan Book", "http://github.com/Grinnz"]
 ---
 
 Perl 5 is a highly capable, feature-rich programming language with over 25 years of development.
@@ -14,7 +15,6 @@ Perl 5 runs on over 100 platforms from portables to mainframes and is suitable f
 ```perl
 # Single line comments start with a number sign.
 
-
 #### Perl variable types
 
 #  Variables begin with a sigil, which is a symbol showing the type.
@@ -37,7 +37,9 @@ 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];
 
 ## Hashes
 #   A hash represents a set of key/value pairs:
@@ -50,11 +52,39 @@ my %fruit_color = (
   apple  => "red",
   banana => "yellow",
 );
+
+# Hash elements are accessed using curly braces, again with the $ sigil.
+my $color = $fruit_color{apple};
+
 # Scalars, arrays and hashes are documented more fully in perldata.
 # (perldoc perldata).
 
-# More complex data types can be constructed using references, which allow you
+#### References
-# to build lists and hashes within lists and hashes.
+
+# 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
 
@@ -105,6 +135,9 @@ for (@elements) {
 # 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
@@ -151,11 +184,96 @@ sub logger {
 # 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;
+
+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;
+
+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.
+
+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;
+
+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.
 ```
 
-#### Using Perl modules
+#### FAQ
-
-Perl modules provide a range of features to help you avoid reinventing the wheel, and can be downloaded from CPAN (http://www.cpan.org/).  A number of popular modules are included with the Perl distribution itself.
 
 perlfaq contains questions and answers related to many common tasks, and often provides suggestions for good CPAN modules to use.