First, if you want to use the term 'geezers', I recommend you don't use nonsense text speak like 'sez'. Us 'geezers' prefer real language :P

Next, on 5.10, your code apparently works for me, without knowing what you're expecting... it runs without error:

This is perl, v5.10.1 (*) built for x86_64-linux

$ perl x.pl
THIRD: 0188 - 0000007777 - 40
THIRD: 0188 - 0000009980 - 41
THIRD: 0123 - 0000008888 - 50
THIRD: 0123 - 0000009999 - 51
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Do you have code that doesn't work? What error do you get when you run it? Perhaps it's due to your example of [a, b, c]. Those are unquoted values you seem to be inserting into an array reference. That won't, and has never worked. Perhaps you meant [qw(a b c)] or [('a', 'b', 'c')], like this (still on 5.10):

use strict;
use warnings;

use Data::Dumper;

my %h;
my $itemArea = 1;
my $itemID = 1;

$h{$itemArea}{$itemID} = [qw(a b c)];

print Dumper \%h;
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Output:

$VAR1 = {
          '1' => {
                   '1' => [
                            'a',
                            'b',
                            'c'
                          ]
                 }
        };
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If the 5.24.x docs literally state: $something = [a,b,c], please let us know where, and I'll patch it.

Also, post your *exact* error, along with the *exact* code that triggers it.