What does it gain, besides a few keystrokes? If the programmer already knows the field names (if he's accessing them via $Username and $Password, it's obvious he does), you're only a few characters shorter than $q->param('Username') or $q->param('Password'). Besides being a big security hole, it doesn't really gain anything and it's less efficient. (Aliasing the variables takes time and memory.)

I'm coming down heavily on the side of the skin-crawlers. That's, at best, a useless technique.