In that case, return a list (or array reference) instead of a rather useless hash reference (as $name is converted into a string which then cannot be used as a sub reference) ...

#  In testpar.pl ...
...
print join ': ' , map $_->() , $objA->parameter( 'a.x' );
...


#  In A ...
package A;
...
sub parameter
{
  ...
  return
    ( \&parameter_name( $param )
    , \&parameter_value( $self->{ values }->{ $para } )
    ) ;
}
...
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