What are you trying to do? If it's trying to wrap the assignment itself in an error checker that throws an exception, you'll have to do something like:

  eval {#$HashOfTNBs{$rec_count}{$subindex} = $1};
  if ($@) {
    throw TNBCriticalColException("Cannot add multiple key entry in ha
+sh array");
  }
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There's no other way around that, since assignment returns the assigned value, rather than a true/false "it worked" value.