%fields was defined as a lexical my variable, so you cannot access it from the symbol table (ie, %{"${class}::fields"}). Define the hash as an our global variable (or use vars qw/%fields/; globals) in both classes and you can do it. This uses a taste of polymorphism (not really true polymorphism) in the new constructor -- no matter what inherited class it was called from, it uses that class's %fields hash.

If you don't like bodging in the symbol table (won't work under strict), it might be smoother would be to have a fields accessor sub in each class that would simply return the %fields hash:

package SomePackage;
my %fields = ( name => undef ); ## can be a lexical

### here's a new simple accessor
sub fields { return %fields };

sub new {
    my $proto     = shift;
    my $class     = ref($proto) || $proto;
    my %args      = @_;

    ### added this line
    my %fields = $class->fields;

    my $self      = {
        _permitted  => \%fields,
        %fields,
    };
    bless($self, $class);
    foreach my $arg (keys(%args)) {
        $self->$arg($args{$arg});
    }
}

## ... elsewhere

package Some::Package::Sub;
use base 'Some::Package'; ### you needed this line before

my %fields = ( name => undef, required => undef );
sub fields { return %fields };
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This uses true polymorphism by calling $class->fields. You can keep the %fields hash as a lexical within the classes as well.

blokhead