#!/usr/bin/env perl

use strict;
use warnings;

#...

sub task_1_a_1 {
    print "Performing task 1 a 1 ...\n";
}

#...

my %stuff = (
    'Major task to perform?' => ['task_1','task_2','task_2'],
    'task_1' => {'Sub-task for task_1?' => ['task_1_a','task_1_b','task_1_c']},
    'task_2' => {'Sub-task for task_2?' => ['task_2_a','task_2_b','task_2_c']},
    'task_3' => {'Sub-task for task_3?' => ['task_3_a','task_3_b','task_3_c']},

    'task_1_a' => {'Sub-sub-task for task_1_a?' => ['task_1_a_1','task_1_a_2']},
    #...

    'task_1_a_1' => \&task_1_a_1,
);

$stuff{task_1_a_1}->();