sub new {
    my $pkg = shift;
    return bless {}, ref $pkg || $pkg;
}
[download]

use strict;
use warnings;

package BaseClass;

sub new {
  my $pkg = shift;
  bless {}, ref $pkg || $pkg;
}

sub bar {
  $_[0]->{'BaseClass::bar'} = $_[1];
}

package SubClass;

our @ISA = qw/BaseClass/;

package main;

my $obj = SubClass->new;

my $foo = $obj->new;

printf "obj isa %s\nfoo isa %s\n", $obj, $foo;
[download]

Using the  bless {}, ref $pkg || $pkg idiom you are writing classes that are safe to subclass.

Do have a look at Damian Conways excellent book Object Oriented Perl

Kay