Reworking your sample code somewhat should help you see the light:

#!/usr/bin/perl
use strict;
use warnings;

package Top;

sub new {
    my ($class, %params) = @_;
    my $self = bless \%params, $class;

    $self->{var1} = 'Top set var1';
    $self->{var3} = 'Top set var3';
    return $self;
}


sub doPrint {
    my ($self) = @_;
    print "Test: $self->{var1}\n";
}


package Top::Extend1;

push @Top::Extend1::ISA, 'Top';

sub doPrint {
    my ($self) = @_;
    print "Extend1 overrides doPrint: $self->{var1}\n";
}


package Top::Extend2;

push @Top::Extend2::ISA, 'Top';

sub new {
    my ($class, %params) = @_;
    my $self = $class->SUPER::new(%params);
    
    $self->{var1} = 'Top::Extend2 set var1';
    return $self;
}

package main;

my $test = Top->new();
my $one  = Top::Extend1->new(something => 'extra');
my $two  = Top::Extend2->new();

$test->doPrint();
$one->doPrint();
$two->doPrint();
[download]

Prints:

Test: Top set var1
Extend1 overrides doPrint: Top set var1
Test: Top::Extend2 set var1
[download]

The main part of the trick is using push @Current::Package::ISA, 'Base::Package'; to hook up the inheritance chain. There are other ways to do that using parent or Exporter, but for light weight OO the push is sufficient.

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