1. Declaring a class
To make a class named in Perl , just save a file 
with ".pm" extention, where we declare a package.

Example::
#!C:/Perl/bin/perl.exe -w
# declaring a package:
#--------------------
package Books;

# Construtor:
#--------------
# The new function (subroutine) constructs an object:
sub new
{
my $class = shift;
my $self = {
# Three attributes
_discipline => shift,
_field  => shift,
_pages  => shift,
};
bless $self, $class;
return $self;
}

# bless() function  returns a reference that becomes an object.

# Define the two helper method Get and Set:
#1. Get (to get the value of a variable):
 

# 1.2. Books discipline

sub get_discipline {
my ( $self) = @_;
return $self->{_discipline};
}

# 1.1. Books field

sub get_field {
my ( $self) = @_;
return $self->{_field};
}

# 1.2. Books pages

sub get_pages {
my ( $self) = @_;
return $self->{_pages};
}


#2.Set (to set a value of a variable):
 
#2.1. Books discipline:

sub set_discipline {
my ( $self, $discipline ) = @_;
$self->{_discipline} = $discipline if defined($discipline);
return $self->{_discipline};
}

# 2.2. Books field:

sub set_field {
my ( $self, $field ) = @_;
$self->{_field} = $field  if defined($field );
return $self->{_field};
}

# 2.3. Books pages:

sub set_pages{
my ( $self, $pages) = @_;
$self->{_pages} = $pages if defined($pages );
return $self->{_pages};
}


#3. Other methods:
# We can other methods, for example:

sub _display {
my ( $self) = @_;
# Print all the values:
print "This is a $self->{_discipline} Books. \n";
print "It is about $self->{_field}.\n";
print "It contains $self->{_pages} pages.\n";
}
#_display();

1;
# The 1; here is important. It is set for confirmation.


#4. Other form for the constructor:

sub new
{
my $class = shift;
my $self = {
_discipline => shift,
_field  => shift,
_pages  => shift,
};
# Printing the values :
print "This is a $self->{_discipline} book. \n";
print "It is about $self->{_field}.\n";
print "It contains $self->{_pages} pages.\n";
bless $self, $class;
return $self;

# ... other functions ...
}


#We can also perm:
print "$class";
$object = new Book( "Physics", "Mechanics", 980);
(constructing an object)