The issue is, what do you think when you see @a[4]?
I think, "The person who wrote this can't be very familiar with Perl."
If I myself were not familiar with Perl, I would probably think the something similar to the person who started this thread: "Huh? I thought you had to say $a[4]! What strange magic is this?"
Maybe it would have been better if the language wasn't this way, but it is, and it's best to use it idiomatically and to urge others to do so as well: if you don't, experienced hackers may mistake you for a beginner, and beginners will find your code confusing.