If your system of equations is of this form:

f'(x) = x * y
g'(x) = x + y;
[download]

You write in your program:

sub dydt {
   my ($t, $x, $y) = @_;
   return ($x * $y, $x + $y);
}
[download]

And then proceed as the example in the docs shows.

Math::RungeKutta doesn't deal with symbolic equations, but expects you to do the evaluation instead.

Thanks! That's what I needed!

The documentation looks reasonable to me. You supply a "function" and a vector of initial values. The evaluation process calls the function you supplied with a time value and a reference to the vector you supplied. Your function should return a list of values corresponding to the values of the vector at the time passed into your function.

If this still doesn't make sense you could show us your function and we may be able to provide sample code.