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

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

Answer 3

In Perl there are some global variables. Here $a and $b are used in the sort function.

I think you might have noticed, like in this statement:

sort { $a <=> $b } @array_name ; 

Answer 4

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)