You could also use a class attribute on Base, rather than an instance attribute:

package Base;
my %_required = ('Foo' => {one =>1, two => 1, three =>1},
                'Bar' => {one =>1, three => 1, fourteen =>1});

sub isRequired {
   my ($self, $attr) = @_;
   my $class = ref $self;
   return exists $_required{$class}->{$attr};
}
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This way, you will store the hash of the required attributes only once.

Greetings, pike