$available_data{'02'}{'03'} = {unit => '02', lesson => '03', next => undef};

$available_data{'02'}{'05'} = {unit => '02', lesson => '05', next => undef};
$available_data{'02'}{'03'}{next} = $available_data{'02'}{'05'};

$available_data{'02'}{'11'} = {unit => '02', lesson => '11', next => undef};
$available_data{'02'}{'05'}{next} = $available_data{'02'}{'11'};

etc...