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
Submitted by: Gea-Suan Lin <gslin at gslin.org>