You're running into another form of autovivification. Try the modifications to your code as listed below.

#!/usr/local/bin/perl -w
 
use Data::Dumper;
 
print "Case 1: non existent\n";
exists $x->{'y'}->{'z'};
print Dumper $x;
 
print "Case 2: undef\n";
$x = undef;
exists $x->{'y'}->{'z'};
print Dumper $x;
 
print "Case 3: hashref\n";
$x = {};
exists $x->{'y'}->{'z'};
print Dumper $x;
 
print "Case 4: stopper scalar\n";
$x = 'stopper';
exists $x->{'y'}->{'z'};
print Dumper $x;
print Dumper \%{$x};
 
print "Case 5: Even assignment won't do, and won't trigger warning or 
+runtime error!\n";
$x = 'stop2';
$x->{'y'}->{'z'} = 'something';
print Dumper $x;
print Dumper \%{$x};
 
print "Case 6: use strict is your friend :)\n";
eval {
  use strict 'refs';
 
  $x = 'stop3';
  $x->{'y'}->{'z'} = 'something else';
  print Dumper $x;
};
 
$@ and print $@;

----------------

Case 1: non existent
$VAR1 = {
          'y' => {}
        };
Case 2: undef
$VAR1 = {
          'y' => {}
        };
Case 3: hashref
$VAR1 = {
          'y' => {}
        };
Case 4: stopper scalar
$VAR1 = 'stopper';
$VAR1 = {
          'y' => {}
        };
Case 5: Even assignment won't do, and won't trigger warning or runtime
+ error!
$VAR1 = 'stop2';
$VAR1 = {
          'y' => {
                   'z' => 'something'
                 }
        };
Case 6: use strict is your friend :)
Can't use string ("stop3") as a HASH ref while "strict refs" in use at
+ ./abcd.pl line 36.
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