A couple of options:

1. Use blessed arrays and constants for your objects. Smaller, faster and safer.
2. Initialise your blessed hashes with the required fields and set them readonly manually.

   use Internals qw(SetReadOnly);;
   $fred = bless {},'fred';;
   @$fred{ qw[ jack john bill ] } = ();;
   
   SetReadOnly $fred;;
   
   print Dumper $fred;;
   $VAR1 = bless( {
                    'john' => undef,
                    'jack' => undef,
                    'bill' => undef
                  }, 'fred' );
   
   $fred->{bill} = 3;;
   print Dumper $fred;;
   $VAR1 = bless( {
                    'john' => undef,
                    'jack' => undef,
                    'bill' => 3
                  }, 'fred' );
   
   $fred->{derf} = 'error';;
   Attempt to access disallowed key 'derf' in a restricted hash at (eval 
   +11) line 1, <STDIN> line 9.
   
   print Dumper $fred;;
   $VAR1 = bless( {
                    'john' => undef,
                    'jack' => undef,
                    'bill' => 3
                  }, 'fred' );
   [download]