keys returns a list of strings, not aliases to the real hash keys. Imagine an array instead of a hash - would you expect the following to work?

# Simulate shift. (Does NOT work!)
for my $i (1 .. $#array) {
     $i--;
}
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To "change" a hash key, you have to delete it and create a new one:

$hash{newkey} = delete $hash{oldkey};
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