$a and $b are special, because you can use them inside a sort (without warnings, with strict active):

use strict;
my @list;
@list = sort {$a <=> $b} @list;
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Offhand I wouldn't know a way to catch the use of undeclared lexical $a and $b, except maybe tieing global $a and $b to something that would bomb if a STORE or FETCH would be done. But that only can work if you don't use sort(), I would think.

Liz