Is there a case that ->can is needed that is not caught by UNIVERSAL::callback ?

No, afaik; it rather catches too much. If some package ought to have a callback method which it hasn't due to some fubar, UNIVERSAL::callback kicks in - long faces, head-scratching. But! if someone writes a method/function for package UNIVERSAL, they should know what they are doing, and my bets are that you do.

perl -le'print map{pack c,($-++?1:13)+ord}split//,ESEL'

In reply to Re^4: Rosetta Dispatch Table by shmem
in thread Rosetta Dispatch Table (Interview Question) by eyepopslikeamosquito

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