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sub fib {
  croak("…")  if @_ != 1;
...
  return $nth if $nth <= 1;
  return fib( $nth - 1 ) + fib( $nth - 2 );
}

my $fib_die_msg = 'fib() takes one positive integer as argument, abort
+ed';
sub fib {
...
  return $nth if $nth <= 1;
  return fib( $nth - 1 ) + fib( $nth - 2 );
}

sub fib {
  my($nth = shift) =~ m{^\d+$} and ! @_ or die $fib_die_msg;
  ...
}

sub fib {
  my $nth = shift;
...
  return $nth if $nth <= 1;
  return fib( $nth - 1 ) + fib( $nth - 2 );
}