my $number_of_true_variables = grep { $_ } $x, $y, $z;

if ( $number_of_true_variables == 1 ) {
    print "Exactly one variable set\n";
}

##</code><code>##

sub number_of_true_variables {
    return scalar grep { $_ } @_;
}

if ( number_of_true_variables( $x, $y, $z ) == 1 ) {
    print "Exactly one variable set\n";
}

##</code><code>##

sub exactly_one_is_set {
    my $number_of_true_variables = grep { $_ } @_;

    return ( $number_of_true_variables == 1 );
}

if ( exactly_one_is_set( $x, $y, $z ) ) {
    print "Exactly one variable set\n";
}