... can we add new methods to a exist object in a package?

Update: WRT choroba's reply here, this reply addresses Perlish OO in general.

Yes, it's a new topic, but...

c:\@Work\Perl\monks>perl
use 5.014;  # needs package NAME BLOCK syntax

use strict;
use warnings;

package Foo {

    use strict;
    use warnings;

    sub new {
        my $class = shift;
        return bless { 'hi' => 'lo' } => $class;
        }

    sub method {
        my $self = shift;
        my ($k) = @_;
        return exists $self->{$k} ? $self->{$k} : 'unknown';
        }

    }

my $of = Foo->new;

printf "A: '%s' \n", $of->method('hi');
printf "B: '%s' \n", $of->method('xx');

printf "C: '%s' \n", $of->oh_by_the_way('hi');
printf "D: '%s' \n", $of->oh_by_the_way('xx');

printf "E: '%s' \n", $of->method('hi');

sub Foo::oh_by_the_way {
    my $self = shift;
    my ($k) = @_;
    $_ .= $_ for values %$self;
    return exists $self->{$k} ? $self->{$k} : $self->method($k);
    }

__END__
A: 'lo'
B: 'unknown'
C: 'lolo'
D: 'unknown'
E: 'lolololo'
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The basic idea is that a subroutine in any package/namespace can be defined from within any other package/namespace if the newly-defined symbol is fully qualified.