You should only use multibyte characters if you are sure that the underlying cryptographic system can handle them. I'm sure it can handle bytes.

And there is a more elegant way, which is not very much different from what you do:

my $salt = pack "C10", map { int(256*rand()) } 0..9

If you want to stick to ASCII (2**70 is still a very large number), just change to constant to 128.