Storable preserves the internal representation of Perl data structures. It is fast, efficient and simple to use.
XML provides a language / platform / application independant method for information (that is - data with structure and contextual meaning associated with it) passing. I suspect that at present, it has none of the three attributes Storable has - implementing an XML solution would result in a program that is slow, inefficient and be complex to write and maintain.
Bottom line: How quickly do you need the solution? If you need it today, stick with storable. Do you need to pass the information to other application? Bite the bullet and learn XML - it is the "solution to all our problems" de jour (sorry for the cynical comment, I just conldn't resist).