package Foo;
use strict;
use warnings;

sub new{
	my $class = shift;
	return bless {foo => 123}, $class;
}

# public
sub foo{
	my $self = shift;
	return $self->{foo};
}

package Foo::Bar;
use strict;
use warnings;
@Foo::Bar::ISA = qw(Foo);

# Overload
sub new{
	my $class = shift;
	return bless {}, $class;
}

package main;

use strict;
use warnings;
use Data::Dumper;

my $foo = Foo->new;
my $bar = Foo::Bar->new;
print Dumper $foo, $bar;
print $bar->foo; # Use of uninitialized value ...