my ( $x_opt, $y_opt, $z_opt );
my $status = GetOptions( x => \$x_opt,
                         y => \$y_opt,
                         z => \$z_opt,
                       );

die $USAGE if !$status ||
              !@ARGV   ||
              ( 1 < grep $_, $x_opt, $y_opt, $z_opt ) ||
              ( $x_opt and @ARGV != 4 ) ||
              ( $y_opt and @ARGV != 3 ) ||
              ( $z_opt and @ARGV != 2 );