In other words, if the item in a Perl symbol table has value undef for all practical purposes is it that same as deleted item in FAT and should be treated by interpreter developers in the same way as "not found" item.

No, that's an incorrect analogy. Assigning undef to a scalar is like truncating a file down to zero bytes - it still exists as before, just its content has been cleared. (Not to mention that lexicals aren't even in the symbol table. See perlmod for the details.)