You need to change the name of your module as it conflicts with the standard module of the same name Test (or use lib, but a name change is probably the more sensible option). Once you've sorted that out you might also want to change your methods to something a bit more sensible like

sub first_name {
  my( $self, $args ) = ( shift, {@_} );
  $self->{ first } = $args->{first};
  return "First Name: $args->{first}\n";
}
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As this way you aren't re-assigning the object's reference (not that it affects the object itself) and you're also assigning the specified arguments passed in to $self so that the full_name method will behave as expected.
HTH

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broquaint