Why doesn't Perl's strictures complain about an undeclared $a?

Question

Why is there no error issued by strict:

use strict;

$a = $a + 1;

Answer 1

$a and $b are special globals used by sort, so they're always defined. Try it with $c instead and you will get an error.

Answer 2

Although strict does not complain about the special $a and $b variables, perlcritic will detect their usage:

Magic variables should be assigned as "local"... (Severity: 4)

Answer 3

$a is a special global variable. It doesn't need to be declared. See perldoc perlvar.