use v5.12;
use Data::Dumper ();

{
	package Backpack;
		
	sub new { # the constructor
		my $class  = shift;
		my %params = @_==1 ? %{$_[0]} : @_;
		
		my $self = bless {}, $class;
		$self->colour($params{'colour'} || 'black');
		$self->contents($params{'contents'} || []);
		return $self;
	}
	
	sub colour {  # a so-called "accessor" method
		my $self = shift;
		$self->{colour} = $_[0] if @_;
		return $self->{colour};
	}
	
	sub contents {  # another "accessor" method
		my $self = shift;
		$self->{contents} = $_[0] if @_;
		return $self->{contents};
	}
	
	sub is_empty {
		my $self = shift;
		return (@{$self->contents} == 0);
	}
	
	sub add_item {
		my $self = shift;
		my ($item) = @_ or die "what item??\n";
		push @{$self->contents}, $item;
		return $self;
	}
	
	sub paint {
		my $self = shift;
		my ($colour) = @_ or die "what colour??\n";
		$self->colour($colour);
		return $self;
	}
	
	sub dump {
		my $self = shift;
		Data::Dumper::Dumper($self);
	}
}

my $x = Backpack->new(colour => "Red");
$x->add_item("keys");
$x->add_item("mobile phone");
$x->add_item("mobile phone charger");

my $y = Backpack->new(contents => ["pants"]);

print $x->dump, $y->dump;