[ Ignore this post and see Re^5: Tie::Hash::MultiValue Unique question instead ]

$hash{$_} = 1 for 2,3,4,5,6,2,7,8;
[download]

and

$hash{$_} = 1 for 2,3,4,5,6,7,8;
[download]

produce the same result, so your request doesn't make much sense.