> My guess is that Mojo::Server::Morbo is loading (the correct) "B.pm"

Good guess! I wrote some test code and used Devel::Modlist to prove to myself that Mojo::Server::Morbo uses B via Future and it ignores the old b.pm in "use lib". No such thing happens at the command line so it loads the old b.pm and fails.

perl -MMojo::Server::Morbo -le 'print $B::VERSION'
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