> What does delete $My::{Run} do?

It deletes the entry for the symbol (typeglob) in the namespaces/symbol table/ STASH %My::

I can't test now, but my guess is that the compiled code is "linked" to the old typeglob's CODE slot. (IIRC that's what B::Concise shows ¹)

Redefining the sub in a require or eval or whatever will repopulate the slot.

But after deleting the entry in the symbol table a new typeglob is created and autovivified into the stash. °

I could create rather complicated tests to prove all of this with all interpretations of what the OP probably wants to achieve.

Unfortunately I'm too busy for guesswork, and honorable ikegami or Dave Mitchell should know the implementation better.

You might be interested into following introspection tools

• B::Terse to scan the OP tree
• ( B::Deparse )
• Devel::Peek
• blessing the coderef into a package with DESTROY

Looking up following documentation might help

• symbol table hash aka STASH
• typeglobs
• import/export mechanisms
• require and eval
• execution phases perlmod
• garbage collection and refcounts
• perlguts
• the Panther book from O'Reilly

Updates

°) Also diving into implementation details is kind of non productive if undef and sub already do the job. And typeglobs are particularly icky Perl4 stuff which are badly explained and supported by introspection.

¹) see Re^2: Refresh a Module