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 };