Did he really ask for undefined methods? Those calls are per definition only resolved at runtime.

Despite manipulating UNIVERSAL in your example, the functions foo() and bar() are still found by B::Xref

$ perl -MO=Xref,-r tst.pl|grep subused
tst.pl           (main)           2 main            & foo             
+ subused
tst.pl           (main)           3 his             & bar             
+ subused
...
[download]

I agree that every approach can be somehow tricked out, but IMHO the vanilla cases are well covered by B::Xref.