From type/Attribute

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method required(Attribute:D: --> Any:D)

Returns 1 for attributes that have the "is required" trait applied, or Mu if the attribute did not have that trait applied. If the "is required" trait is applied with a string, then that string will be returned instead of 1.

class Library {
    has $.address is required;
    has @.new-books is required("we always need more books");
}
my $addr = Library.^attributes(:local)[0];
my $new-books = Library.^attributes(:local)[1];
say $addr.required;      # OUTPUT: «1␤» 
say $new-books.readonly; # OUTPUT: «"we always need more books"␤»