"could have been stated in just this way: %fields = { id => 0, name => 1, age => 2};"

Assign a key/value pair to a hash:

%hash = ( key => "value" );
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Assign a hashref as a hash key with no value:

%hash = { key => "value" };
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perl -MData::Dumper -wE 'my %fields = { id => 0, name => 1, age => 2};
+ say Dumper \%fields;'
Reference found where even-sized list expected at -e line 1.
$VAR1 = {
          'HASH(0x7f9d5a804850)' => undef
        };
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could have been stated in just this way: %fields = { id => 0, name => 1, age => 2};
...
you must check for correctness v-e-r-y carefully

People are constantly downvoting your posts? That's why.

In my opinion, the first code example is unnecssarily complicated – notice that it “sent you to Camel book, to no avail.” It all comes down to this, and could have been stated in just this way: %fields = { id => 0, name => 1, age => 2};

It all comes down to, the information the OP already provided?

How can you be confused about what the OP knows when you're actively copy/pasting what he knows?

Calling a hash slice "unnecssarily complicated" is very silly and very telling

because future bug-sleuths will naturally tend to read the human comment and not verify that the code in fact matches it.)

Thats what non-programmers naturally do because they can't read code

Programmers would ignore the comments and read the code

I was going to use a foreach loop, but the idiomatic code seemed (and now that I understand it, IS) better for maintainability, given that it hids indexing and is very concise

Obviously I needed to understand its workings before employing it en masse; now that it is clear to me I can say that these are the kind of things that make me love perl and I have a new nifty piece of syntax in my arsenal.

As for the $test variable, I agree that I could've chosen a better name (maybe $constraints like in the python original code). And given that it is just a one-off script made for fun, it is overkill to use a variable to hold the boolean, but I was influenced by habit (keep complex boolean conditions outside of the if) and by the over complex python code, where they actually create a function for every constraint and then put them in a tuple to map them to each permutation with all().

Thank you for your observations.