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.

WWW: http://search.cpan.org/dist/Array-Unique/