This problem is not necessarily a simple one. Depending on the scale of the map, and the map projection used (the map is flat, the earth isn't), your task may be a relatively simple one or virtually impossible.

For starters, check out the Geo::Coordinates modules. You can convert the lat/lon to an appropriate UTM mapping given an ellipsoid. Then, as kyle points out, you will need to know how many metres to the pixel, or perhaps the lat/lon of another point on the map. With just one point and no scale, you cannot do what you want.