use warnings;
use strict;
use Test::More tests=>2;

sub getsubset {
	...
}

my $data = [
	['a','b','c','d'],
	['e','f','g','h'],
	['i','j','k','l'],
	['m','n','o','p'] ];

my $range = 'R2C2:R3C3';  # or any other useful syntax!
my $subset = getsubset($data, $range);

is_deeply $subset, [
	['f','g'],
	['j','k'] ];

# Part 2:
$subset->[0][1] = 'X';
$subset->[1][0] = 'Y';

is_deeply $data, [
	['a','b','c','d'],
	['e','f','X','h'],
	['i','Y','k','l'],
	['m','n','o','p'] ];