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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 }

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

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In reply to Re^3: Eliminating "used only once" warnings from List::Util::reduce by BrowserUk
in thread Eliminating "used only once" warnings from List::Util::reduce by davido

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    [1nickt]: choroba Understood. I'm wondering about the logic in a cpanfile. If the perl doesn't support threads, it's easy: require MCE. If the perl does support threads, as you say the user has a choice, so require both? Or, assume that irrespective of the choide
    [1nickt]: ... choice, the threaded-perl user *probably* wants to use threads, therefore only require threads and leave MCE out, allowing the user to manually install if desired?
    [choroba]: just require any of MCE and threads. Not sure if that's possible in the cpanfile, but should be possible early in the Makefile
    [1nickt]: The app expects threads as default anyway, right?
    [choroba]: that's true
    [1nickt]: expects threads *to be* the default
    [1nickt]: I think cpanfile can handle the logic. testing now.

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