One way to silence the warnings is to disable them in a localized scope: 
use warnings;
use strict;

use List::Util qw/reduce/;
{
    no warnings qw(once);
    print reduce { $a * $b } 1..6;
}

[download]

Update: added "once"

Update: Refer to the following discussion: https://rt.cpan.org/Public/Bug/Display.html?id=65629

In reply to Re: Name "main::a" used only once: possible typo by toolic
in thread Name "main::a" used only once: possible typo by szabgab

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