#!/usr/bin/perl

use warnings;
use strict;

package A {

    sub new {
        my $classname = shift;
        my $self = {};
        $self->{b} = B->new($self);
        return bless($self, $classname);
    }

   sub say_hello {
        my $self = shift;
        print "Hello! Args: ". join(',', @_) ."\n";
    }

    sub pass_sub {
        my $self = shift;
        $self->{b}->use_sub_later();
   }
}

package B {

    sub new {
        my $classname = shift;
        my $a = shift;
        my $self = {};
        $self->{a} = $a;
        return bless($self, $classname);
    }

    sub use_sub_later {
        my $self = shift;
        my $ready_to_use = 1;
        if ($ready_to_use) {
            $self->{a}->say_hello('ARG1', 'ARG2');
        }
    }
}

my $a = A->new();
$a->pass_sub();