Thank you ! It is always a pleasure to see your replies.

But it is a bad practice to use a key in hashes without quotes, as in $main::{X} ;-)

A more revealing variant of your test would be 

[0]  # cat undef_exists_test.pl
use v5.10;
say $main::{'X'};
say $X;
say $main::{'X'};
if( exists $main::{'X'} ){ say "exists";} else {say "does not exist";}
if( defined($X) ){say "defined";} else {say "not defined";}

$X=1;
undef $X;
if( exists $main::{'X'} ){ say "exists";} else {say "does not exist";}
if( defined($X) ){say "defined";} else {say "not defined";}
Running it I got: 
*main::X

*main::X
exists
not defined
exists
not defined

[0]  #

We can see that even before the statement "say X;" executed the variable X was already added to the symbol table (during begin block execution?),

So it does exists.

NOTE:

I think undef is not a value, as many here assume, but a function, So 
$X=undef;
actually means in Perl: 
$X=undef();
and is equivalent to 
undef $X;

In reply to Re^4: Two meanings of undef by likbez
in thread Two meanings of undef by likbez

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