#This says "Not a CODE referece at Foo.pm line..."
  $self->{funcs}{correlation} = \&$self->correlation;

  #This says "Can't call method 'correlation' on unblessed
  # reference at Foo.pm line..."
  $self->{funcs}{correlation} = \&{$self->correlation};

  #And this says "Scalar found where operator expected at
  # Foo.pm line...
  $self->{funcs}{correlation} = \&($self->correlation);