Hello Great Monks! Could anyone explain why this program dies? "x" valu in hashref $h shouldn't be modified... in the same manner as "y" was not modified...

#!/usr/bin/perl -CSDA

use utf8;
use Modern::Perl qw{2017};

use Data::Dumper;
$Data::Dumper::Sortkeys = 1;

my $h = {a => 1, b => "", c => 2};
my $d1 = Dumper($h); 

printf "grep: %s\n", (join " ... ", grep { $_ }  $$h{a}, $$h{x}, $$h{b
+}, $$h{c});

printf "test: %s\n", $$h{y} ? "yes" : "no";

my $d2 = Dumper($h); 
$d1 eq $d2 or  die "\nFAIL! h{x} modified, h{y} not modified... why?\n
+\n$d1\n$d2";
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result:

grep: 1 ... 2
test: no

FAIL! h{x} modified, h{y} not modified... why?

$VAR1 = {
          'a' => 1,
          'b' => '',
          'c' => 2
        };

$VAR1 = {
          'a' => 1,
          'b' => '',
          'c' => 2,
          'x' => undef
        };
[download]