You are viewing the version of this documentation from Perl blead. This is the main development branch of Perl. (git commit 05e26d5a81052fe889283096dc4fe5b8f95ce1a9)
$a
$b

Special package variables when using sort(), see "sort" in perlfunc. Because of this specialness $a and $b don't need to be declared (using use vars, or our()) even when using the strict 'vars' pragma. Don't lexicalize them with my $a or my $b if you want to be able to use them in the sort() comparison block or function.