The following also works, though is equally surprising that it is needed.

my $lib;
use lib $lib = '.';
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This works because variables declared at one scope are accessible from scopes inside the declaring scope.

Unfortunately, there isn't a good way to change this. It can, and probably should be, documented in the page for use and, maybe, also in the page for the lib pragma.

(Maybe a warning could be added when parsing use to remind people that my inside use has no effect and suggest alternate syntax.)