Some quick comments:

• You should look at using return rather than using a global variable to return results from a function.

• Just like you need to learn how to return values to a function, you need to learn how to pass arguments to a function (such as $coin to pence).

• Your teacher or teaching material is giving you some bad advice if it advocates using & on function calls. The & is not just decorative.

• use strict; will reveal many errors, easily fixable by using my. You should be using use warnings; as well.