"name used only once" warning while using List::Util::reduce

Question

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How do I prevent List::MoreUtils from warning about using $a and $b only once?

I get a warning like this:

Name "main::a" used only once: possible typo at analysis.pl line 15.

on this line

return join("_", @monsters) if @monsters && List::Util::reduce { $a && $b // 0 > 0 } 1,@monsters;

I understand that $a and $b are not used anywhere else (and why it would be a problem for an ordinary variable), but I'd like to get rid of the warning - after all, it is the List::Util::reduce function that will set $a and $b. It is similar to sort function.

Please, don't suggest turning warnings off. This is not something I want to do.