Name "main::a" used only once: possible typo

szabgab has asked for the wisdom of the Perl Monks concerning the following question:

use strict;
use warnings;

use List::Util qw/reduce/;
print reduce { $a * $b } 1..6;
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Genetates the following warnings:

Name "main::a" used only once: possible typo at x.pl line 6.
Name "main::b" used only once: possible typo at x.pl line 6.
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OTOH

use strict;
use warnings;

print sort {$a <=> $b } 1..6;
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does not generate the warnings. As expected. (running perl 5.14.2 and List::Util 1.27)

Why did the first example generate the warnings?
What is the recommended way to silence them?
Why is this not mentioned in List::Util?