Actually, in my example I forgot to delete $INC{"My.pm"}; so the second require wasn't redefining My::Run. When I add the delete it works as expected, printing: 
Loading...
a: Running
Loading...
b: Running

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In the OPs example there is also a delete $My::{Run}. So 
require My;
print "a: "; My::Run(); # a: Running
undef &My::Run;
delete $INC{"My.pm"};
delete $My::{Run};
require My;
print "b: "; My::Run(); # Undefined subroutine &My::Run called at ...

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says My::Run() is undefined in the second call. What does delete $My::{Run} do?

In reply to Re^4: Refresh a Module by Danny
in thread Refresh a Module by Timka

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