Will this create the keys {$G_ExcepId}{$G_CauseCode} also ?

It's not the keys you should be worried about, but the values (the hash refs) at those keys. And yes, those are created. And thus, so are the keys.

$HoH{"$G_Boardno"}{"$G_ExcepId"}{$G_CauseCode} = "value";
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is short for something like

$HoH{"$G_Boardno"} //= {};
$HoH{"$G_Boardno"}{"$G_ExcepId"} //= {};
$HoH{"$G_Boardno"}{"$G_ExcepId"}{$G_CauseCode} = "value";
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due to autovivification. When an undefined value is derefenced, the appropriate value is created.

>perl -wle"my($x,$y); $y=$x->{k}; print $x;
HASH(0x225278)

>perl -wle"my($x,$y); $y=$x->[0]; print $x;
ARRAY(0x225278)
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