Fix data race in t/pod-valid.t
Fix data race in t/pod-valid.t

Seem to produce floppy failures. No idea is this fixes them or not,
but fairly sure there's a data race with multiple Proc's trying to
create new keys in the same hash.

perl6/doc,stmuk/doc,tbrowder/doc,perl6/doc,jonathanstowe/doc,tbrowder/doc,dmaestro/doc,perl6/doc,dmaestro/doc,tbrowder/doc,perl6/doc,dmaestro/doc,jonathanstowe/doc,MasterDuke17/doc,dmaestro/doc,MasterDuke17/doc,stmuk/doc,jonathanstowe/doc,stmuk/doc,jonathanstowe/doc,jonathanstowe/doc,perl6/doc,MasterDuke17/doc,MasterDuke17/doc,dmaestro/doc,stmuk/doc,tbrowder/doc,stmuk/doc