Yes, the reason it happens after the BEGIN stage rather than during, when most compile-time warnings are issued, is that Perl needs to check the whole lexical scope (i.e. file probably) before it can issue any warnings about variables being used only once. (Because until it's got to the end of the scope it cannot be sure they were only used once.)

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

In reply to Re^3: 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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