# module

sub perform {
    my ($x, $y) = @_;

    if ($x < 10){
        return $x + $y;
    }
    if ($x == 10){
        return $x * $y;
    }
  
    return $x - $y;
}

##</code><code>##

    is perform(5, 5),  10, "...";
    is perform(10, 5), 50, "...";
    is perform(20, 5), 15, "...";