package My::Class;
use strict;
use warnings;

{ 
    # hide from the rest of the class
    my %CLASSDATA = ( foo => 1, bar => 2, baz => 3 );

    # install generic accessor/mutator methods
    no strict 'refs';
    for my $key(keys %CLASSDATA) { 
        *$key = sub { shift; 
                      $CLASSDATA{$key} = shift if @_; 
                      return $CLASSDATA{$key}; 
                     };
    }
    use strict 'refs';
}