G'day szabgab,
I see you've got explanations for why this is happening. In terms of a workaround, I would probably add $a || $b || 1; after the use List::Util ... line. This would help to indicate why that code was there through association and would also remove the clutter of no warnings ... or our ... from the calls to reduce(). Furthermore, changes to code would not require any additional workaround changes. Here's some examples:
$ perl -Mstrict -Mwarnings -E '
use List::Util qw/reduce/;
say reduce { $a * $b } 1..6;
'
Name "main::a" used only once: possible typo at -e line 3.
Name "main::b" used only once: possible typo at -e line 3.
720
$ perl -Mstrict -Mwarnings -E '
use List::Util qw/reduce/; $a || $b || 1;
say reduce { $a * $b } 1..6;
'
720
$ perl -Mstrict -Mwarnings -E '
use List::Util qw/reduce sum/; $a || $b || 1;
say reduce { $a * $b } 1..6;
say sum 1..6;
'
720
21
