Your test code defines the constant BIRD differently from the code you initially ask about; perhaps that explains the difference in behavior?

Thank you for pointing this out, but unfortunately it does not. The mistake was done when I edited the posting after having pasted the test code. Here is the original testcode - if you try it out, you will see that it shows the behaviour I was talking about in my post.

use strict;
use warnings;
use Data::Dumper;
use constant { BIRD => 'footchicken' };
print Dumper(+{ +BIRD => 'Valery' });
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In my original posting, I used "=" instead of "=>" in one place (I have this corrected now). This code, as posted here, wouldn't even have compiled.