If sort BLOCK LIST is encountered in a package, Perl switches off "used only once" warnings for the $a and $b package variables.

use warnings;
use List::Util;
my $reduced = List::Util::reduce { $a + $b } 1, 2, 3, 4;
print "$reduced\n";
()=sort{;}();

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... that's why sort is magic.

List::Util could easily eliminate the warning; and it doesn't even need to use XS trickery to do so. It just needs to add something like this to its import method:

my $pkg = caller;
eval qq[ package $pkg; our \$a; our \$b; ];

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I believe the old pure Perl implementation of List::Util used to do something along these lines. It seems the shiny, new XS-only version does not. I'd count that as a regression.

package Cow { use Moo; has name => (is => 'lazy', default => sub { 'Mooington' }) } say Cow->new->name

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

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