What's the point of `use vars` in this Perl subroutine?

Question

In one of the chapters in Mastering Perl, brian d foy shows this snippet from List::Util:

sub reduce(&@) {
    my $code = shift;
    no strict "refs";
    return shift unless @_ > 1;
    use vars qw($a $b);
    my $caller = caller;
    local(*{$caller . "::a"}) = \my $a;
    local(*{$caller . "::b"}) = \my $b;
    $a = shift;
    foreach(@_) {
        $b = $_;
        $a = &{$code}();
    }
    $a;
}

I don't understand what's the point of the use vars qw($a $b) line. Even if I comment it, I get the same output & warnings.

Answer 1

This is done because List::Util uses reduce() function internally.

In the abscence of use vars, the following warning is given when the function is used:

Name "List::MyUtil::a" used only once: possible typo at a.pl line 35.
Name "List::MyUtil::b" used only once: possible typo at a.pl line 35.

You can see this for yourself by running the following code:

use strict;
use warnings;

package List::MyUtil;

sub reduce (&@) {
   # INSERT THE TEXT FROM SUBROUTINE HERE - deleted to save space in the answer
}

sub x {
    return reduce(sub {$a+$b}, 1,2,3);
}

package main;
my $res = List::MyUtil::x();
print "$res\n";

And then running it again with use vars disabled.