Really? Sure, with warnings, you get a warning message. 
However, the program produces what it should produce, so the 
warnings generated are a nuisance. 

Perhaps you say "yeah, I know, but golly, perhaps in the 
future '@_[$a]' will mean something else". But that, I don't 
believe. In fact, in the future, in perl6, '@_[$a]' will be
the correct way and '$_[$a]' will be wrong way. I don't think 
anyone will give '@_[$a]' a meaning other than '$_[$a]' in 
perl 5.12, or another perl5 version. Ever.

(As for why this is all in 'code' tags, I've used [] several 
times in the posts. It's too much of a hassle to type 13
characters for each occurance)