I certainly try not to use too many temporary, intermediate variables, but I do if they add to readability for a future maintainer of the code.

My basic criteria for using a temporary variable is this:
If I can think up a meaningful, transparent name for it, I can use it.

For example, in your case, @chars and $salt are both meaningfully named, thus allowed. What I would not allow would be @array and $foo for the same variables.

Regards,
Helgi Briem