Thanks to you. It is a really nice reminder about how to find a minimum but unfortunately, I cannot use it directly. I guess I haven't defined my problem well enough. I have a list of x and y, not the equation it represents. So I have to find it. Math::polynomial does the job for me and it also gives me the extremum. Which is exactly what I need. So, I need to resolve the equation y = polynomial to get the x that matchs the y found. But, then againg, I think I have asked too soon, I did finally found a module who looks like it can solve my problem (with that dear google friend of mine) : Math::GSL::Polynomial.