I was going to post the same thing, until I created a test script with the OP's example data. The subsequent error message led me to the docs, where I found this (emphasis mine):

If a string is given, it is taken to be a run list. A run list specifies a set using a syntax similar to that in newsrc files. A run list is a comma-separated list of runs. Each run specifies a set of consecutive integers.

Restrictions: The runs in a run list must be disjoint, and must be listed in increasing order.

Since the OP's example data is not ordered, a solution like the ones provided above is necessary before Set::IntSpan can be used. This, however, makes Set::IntSpan moot for the purpose of the OP. :-)