You will find it helps in many ways (not least, you may solve the problem yourself) if you provide a small code sample that demonstrates the issue. Tell us what you expect to happen and what you think is happening. If you have a good case and a bad case, show data for both.

By the way, you almost never need to interpolate variables into a string as the only content of the string. Also it would help you a lot to use a little white space so you can easily identify the "phrases" in your "sentence". Rewritten with those two points taken into consideration gives:

$HoH{$G_Boardno} = {$G_ExcepId => "{$G_CauseCode=>value}"};
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where it is much easier to see that "{$G_CauseCode=>value}" is a string with a variable interpolated into it and not a nested hash reference as you probably expected.