From language/typesystem

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multi sub trait_mod:<is>(Attribute $attr, :$required!)
multi sub trait_mod:<is>(Parameter:D $param, :$required!)

Marks a class or roles attribute as required. If the attribute is not initialized at object construction time throws X::Attribute::Required.

class Correct {
    has $.attr is required;
}
say Correct.new(attr => 42);
# OUTPUT: «Correct.new(attr => 42)␤» 
 
class C {
    has $.attr is required;
}
C.new;
CATCH { default { say .^name => .Str } }
# OUTPUT: «X::Attribute::Required => The attribute '$!attr' is required, but you did not provide a value for it.␤»

Note a class with a private attribute will give the same error:

class D {
    has $!attr is required;
}
D.new;
CATCH { default { say .^name => .Str } }
# OUTPUT: «X::Attribute::Required => The attribute '$!attr' is required, but you did not provide a value for it.␤»

You can provide a reason why it's required as an argument to is required

class Correct {
    has $.attr is required("it's so cool")
};
say Correct.new();
# OUTPUT: «The attribute '$!attr' is required because it's so cool,␤but you did not provide a value for it.␤»