I haven't done any of this since university but I remember that a spline can be used to calculate the points on a line given a number of existing ordinates. I forget how it works (I learned about it for an exam and then forgot it pronto) but a quick CPAN search yielded Math::Spline which appears to do the trick.

according to the POD, you can create a Math::Spline object with a number of points (as two array refs). You can then evaluate the result for a particular point.

The advantage of this method is that you can locate a point that is on a continuous line but which isn't a point in your data.