In FAT if you delete item from the directory it is still present but marked as deleted.

So the fact that it is still present does not change the fact that semantically it is deleted.

Further to stevieb's and haukex's comments:   In a file system, if you delete a file and then create another file with the same name, do you get any warning that the old file still, in any sense, "exists", or "masks" anything, or that there is any possible symbol/name collision? The symbol spaces of Perl are not file systems.

c:\@Work\Perl\monks>perl -wMstrict -le
"my $scalar = 42;
 undef $scalar;
 my $scalar;
"
"my" variable $scalar masks earlier declaration in same scope ...
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I think the critical point that you miss is that undef is a very well-defined value! It is not random, it can be tested, scalars having that value exist in every sense, etc.