$HoH{"$G_Boardno"}{"$G_ExcepId"}{$G_CauseCode} = "value";

##</code><code>##

$HoH{"$G_Boardno"} //= {};
$HoH{"$G_Boardno"}{"$G_ExcepId"} //= {};
$HoH{"$G_Boardno"}{"$G_ExcepId"}{$G_CauseCode} = "value";

##</code><code>##

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

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