If you are talking about the 'my' before the $final_href on line 7, it was by mistake, but it is NOT the issue I was facing.

For reasons of practicality that doesn't matter. When you post code (which should be an SSCCE but in your case is not) which contains such mistakes and they are pointed out to you and you ignore them it does not encourage anyone else to try to help you. If you haven't even been bothered to fix the bugs which you can then why do you expect anyone to help you fix the bugs which you cannot?

I do understand the problem, I know why it occurs and where I just don't understand how to solve it.

Then why isn't that the code you have shown, as an SSCCE so that it is then obvious where the problem lies? Take a moment to read that link and understand that all the monks have only seen a little bit of your code which, in isolation and being incomplete, makes it very difficult to debug. Read also the excellent How to ask better questions using Test::More and sample data to improve the problem descriptions even further.

Anyway, I feel like you and especially LanX, are not so nice.

And there's a perfect way to alienate a community of experienced Perl hackers who would otherwise give you the benefit of their wisdom for free. Recant, brother, while you yet may.

%{ $files_href->{data} // {} }
[download]

Thank you. I didn't know there is a special operator like this one. Another small question if you may. Is it possible to insert the data into two hash at the same line? something like:

        (%{$one},%{two}) = 
        (
            data => { ( %{ $data{data} }, %{ $one->{data} // {} } ) },
            total => do
            {
                my %total;
                for my $href ( $one->{total}, $data{total} ) {
                    $total{$_} += $href->{$_} for keys %{ $href };
                }
            \%total;
            }
        );
[download]

Only with newer Perl versions.

https://www.effectiveperlprogramming.com/2015/08/create-named-variable-aliases-with-ref-aliasing/

Cheers Rolf
_{(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery FootballPerl is like chess, only without the dice}

Sorry, didn't intend to be "not so nice". When debugging problems remotely, and aren't intimately acquainted with the goal, people in help forums often focus on the first issues we see, even if they aren't the root cause. Moreover, it seemed reasonable that that it was your root cause (though it turned out not to be) because the new as-of-yet unassigned $files_href created by the second 'my' would have started as undefined, so it appeared that was why it was trying to use an "undefined value as hash reference". I saw what LanX was hinting about, and agreed that it was a good first step in the debugging process for you; since your post indicated you hadn't seen the 'my' issue yet, I tried to clarify, to further help you along toward fixing that part of the issue.

I guess "staring you in the face" might have been harsh -- I didn't intend it to be. Sorry

So why do you answer like this? during the conversation, you made me feel like the stupidest man alive because I didn't know the existence of the // operator.

Our goal is to help people to learn programing, and you are constantly ignoring our advice.

And we work hard to give you excellent advice which is neglected.

Cheers Rolf
_{(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery FootballPerl is like chess, only without the dice}