You might find fields useful, although you'd have to create an object instead of a plain hash, and you'd need a different class for each hash.

{ package FooBar;
  use fields qw( foo bar );
  sub new { fields::new(shift) }
}

my $ref = new FooBar;

$ref->{foo} = 1; # Okay
$ref->{quv} = 2; # Kablooie!
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I'd guess that with Symbol one could create a class factory that would return a singleton object with the fields you request.