This package lets you create an array which will allow only one occurrence
of any value.
In other words no matter how many times you put in 42 it will keep only
the first occurrence and the rest will be dropped.
You use the module via tie and once you tied your array to this module it
will behave correctly.
Uniqueness is checked with the 'eq' operator so among other things it is
case sensitive.
As a side effect the module does not allow undef as a value in the array.