$HoH{"$G_Boardno"}{"$G_ExcepId"}{$G_CauseCode} = "value";
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Will this create the keys {$G_ExcepId}{$G_CauseCode} also ? As its an empty hash before this line:

 %HoH = () ;
$HoH{"$G_Boardno"} = { "$G_ExcepId" => "{$G_CauseCode => value}" };
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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}

You could definitely say for sure on your own with Data::Dumper. Check it out.

-Paul

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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Yes it will. You will get value by $G_Boardno which is hash from where you get value by $G_ExcepId which is also hash and so on. You can continue like this creating more and more levels until memory runs out.
It is equal to :

$HoH->{$G_Boardno}->{$G_ExcepId}->{$G_CauseCode} = "value";
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Regards,
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Yes, it is called autovivification (see here Re: Complex Data Structure)

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.

#!/usr/bin/perl
use warnings;
use strict;
my %HoH;
my $value = "I have a value!";
my $G_Boardno = "curiosity";
my $G_ExcepId = "killed_the";
my $G_CauseCode = "cat";

$HoH{"$G_Boardno"}{"$G_ExcepId"}{$G_CauseCode}="$value";
print "My hash = ", $HoH{curiosity}{killed_the}{cat}, "\n";
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C:\Code>perl hohohoh.pl
My hash = I have a value!
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