I am reading this document to learn Perl's taint mode => <http://www.webreference.com/programming/perl/taint/index.html>

It is mentioned one way of clean tainted value,

> 
> Another more obscure way to clean tainted values is to use them as a
>  hash key; since hash keys themselves are never considered tainted
> 
> 
> 

I do not quite understand what means "use them as a hash key", and why hash key as never treated as tainted. Appreciate if anyone could help?

thanks in advance,
Lin