About your downsides...

1) Yup, I can see capturing the 62-element in a closure instead of re-creating it. I didn't think of that when I was coding off the top of my head.

2) Nice catch

3) Oops... I missed that. I looked in the library to verify the result of rand() and it said it returns a float, not an int. But I guess the [ takes care of that...

Since this is supposed to be a "UniqueID" and not a password, using "pronounceable" passwords are probably not necessary.

Why put it in a BEGIN block? Doesn't seem necessary to me.