BEGIN { %Foo::FIELDS = qw(a 1 b 1) };
my Foo $x = {};
$x->{c} = 1; # compile-time error
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Dave.

Interesting. I guess this explains the limitation with Lexical::Types where it allows you to define, say, my Str $x to do something else other than check $x is a blessed Str object (and I've started to explore that possibility using Lexical::TypeTiny), but still requires there to be a Str package defined. I guess Perl needs that package to exist so that it can look for %FIELDS.

toby döt ink