Yes, almost always, @foo[$bar] is broken, and should have been written $foo[$bar]. In many cases, this is an undetectable mistake, as in:

$total = @a[3] + @a[4]; # works
print @a[5]; # works
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But is fatal when we use it in a place where it makes a difference:

@a[3] = <STDIN>; # read one line?  NOPE!
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Hence the very right warning.

-- Randal L. Schwartz, Perl hacker