I think a better solution would simply be for Perl to completely ignore $a and $b with respect to "used only once" warnings.

The biggest problem is: which $s & $b?

That is, it is fairly easy to preempt the warnings for $main::a & $main::b -- by soemthing as simple as BEGIN{ $main::a || $main::b } somewhere in List::Util.pm; -- but if reduce() is called from some other package they won't be the right ones. Ie. If reduce() is called from package fred,then it'll be $fred::a & $fred::b.

And there is no way to determine which package you were called from at BEGIN time. With this added to List::Util.pm:

BEGIN{
    printf "'%s'\n", scalar caller();
    $main::a || $main::b
}
[download]

The current package at BEGIN time is main for both of these:

C:\test\primes>perl -Mstrict -MList::Util=reduce -wE" say reduce{ $a +
+$b } 1,2,3"
'main'
6

C:\test\primes>perl -Mstrict -wE"package fred; use List::Util qw[ redu
+ce ]; say reduce{ $a +$b } 1,2,3"
'main'
Name "fred::a" used only once: possible typo at -e line 1.
Name "fred::b" used only once: possible typo at -e line 1.
6
[download]