If I understand correctly, you want to use $destref to modify your hash. Perhaps what you are looking for is illustrated in the following session under the debugger:

  DB<1> %testHash =();

  DB<2> $hashRef = \%testHash;

  DB<3> $destref = \${$hashRef->{'a'}};

  DB<4> x \%testHash;
0  HASH(0x302ee0c8)
   'a' => SCALAR(0x302ee218)
      -> undef

  DB<5> $$destref = 42;

  DB<6> x \%testHash;
0  HASH(0x302ee0c8)
   'a' => SCALAR(0x302ee218)
      -> 42
[download]

As you can see, assigning 42 to $$destref does the trick. But is this what you are trying to do? Hum, probably not, because you have here one extra level of indirection that you probably don't want.

So, maybe, you rather want something like this:

  DB<1>  %testHash =();

  DB<2> $destref = \$testHash{'a'}

  DB<3> x \%testHash;
0  HASH(0x302ee0c8)
   'a' => undef
  DB<4> $$destref = 42;

  DB<5>  x \%testHash;
0  HASH(0x302ee0c8)
   'a' => 42
[download]

My gut feeling (if I understood correctly) is that this is more probably what you are trying to do.