Expanding on afoken's explanation:

Your declaration of

use My::Foo 1.0;
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is looking for the subroutine My::Foo::VERSION
which does not exist (because your package name is Foo, not My::Foo).

Update: The VERSION subroutine , if absent, is implicitly imported from UNIVERSAL - into your package, which , again, is named "Foo" and not "My::Foo" - hence "My::Foo::VERSION" continues to not exist, whereas Foo::VERSION would exist as a result of "package Foo;".