The best way to do this is probably to just inherit accessor and mutator methods, and not try to alter the class data directly.
package Parent;
our $knob = 1;
sub knob {
my $class = shift;
$knob = $_[0] if @_;
return $knob;
}
package Child;
use base 'Parent';
# inherits knob method
That way, calling Parent->knob or Child->knob will get and set the same package global in Parent. To make it safer, you could make $knob a lexical, so it's only visible via the method call.
Update: Better mutator code per Tanktalus's comment.