my $fib_die_msg = 'fib() takes one positive integer as argument, aborted';
sub fib {
  my($nth = shift) =~ m{^\d+$} or die $fib_die_msg;
  @_ and die $fib_die_msg;
  return $nth if $nth <= 1;
  return fib( $nth - 1 ) + fib( $nth - 2 );
}