$a and $b

$ and $b are special package variables when using sort(). 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.

For further study: perldoc -f sort