If the function return undef, I've got the warning "Odd number of elements in hash assignment".

Yeah, that is kind of irritating isn't it? Probably there is a good reason.

You might want to investigate the use of eval...but also notice that this is allowed, even if %hash is "undef":

if (defined($hash{non-existant key})) ...
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the condition, of course, will be untrue, unless there really is a key called "non-existant key" and it is defined.