Hello aral,

I do not even know whether the "problematic line" is able to create a valid reference, because element 'a' does not exist at the time.

It is able to do this, as you can see by dumping the contents of %testHash immediately afterwards:

#! perl
use strict;
use warnings;
use Data::Dump;

my %testHash;                # initialize hash
my $hashRef = \%testHash;    # get a reference to that

# get a reference to an element 'a' inside the hash (that does not yet
+ exist)
my $destref = \${$hashRef->{'a'}}; # <- problematic line

dd \%testHash;
[download]

Output:

 2:28 >perl 1389_SoPW.pl
{ a => \undef }

 2:28 >
[download]

This occurs because of autovivification, an important feature of Perl. See the tutorial “Autovivification: What is it and why do I care?” by Uri Guttman.

Hope that helps,