Your code does not do what you think. Look at perldsc and Data::Dumper to dump out the data structure you're creating.

"{$G_CauseCode => value}"
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is a string, not a hash reference. So your code

$HoH{"$G_Boardno"} = { "$G_ExcepId" => "{$G_CauseCode => value}" };
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creates at the second level a hash mapping of the value of $G_ExcepId to the string {... => value}