in reply to
Is modifying the symbol table to redefine subroutines evil?
Modifying the symbol table can be useful: at work we used a similar trick to write Test::Resub. (Its use is slightly different from what you're doing, but you might like to give its source a quick read.)
If call_me_only_once() really should only be called once, then why not solve the problem there instead of in its caller? (You've already seen how to enforce single execution by scoping a variable outside the sub.)
Addressing the problem there would mean you wouldn't need to worry about bar() making a call to call_me_only_once(); presumably your program knows the answer to that question already.