The article you'll want to read is perllol. The primary issue you are having is that multi-level data structures are formed using the flat data structures combined with references to other data structures - Perl just does it explicitly. The syntax you want in order to create your hash is probably:

%Data_Hash = (
    Call_Hash => {
            TST_IDS_Large_1 =>2.9E-12,
            TST_IDS_Large_2 =>1.85E-12,

            TST_IDS_Small_1 =>1.86E-9,
            TST_IDS_Small_2 =>2.83E-9,
        },
    OPT_VGS_1 =>0,
    OPT_VGS_2 =>0.02,
    
);
[download]

{key1 => val1, key2 => val2} returns a scalar pointer to a hash with the key value pairs listed. In order to interact with it, your code would read:

@idlarge = ($Data_Hash{Call_Hash}{"TST_" . "IDS_Large_1"}, $Data_Hash{Call_Hash}{"TST_" . "IDS_Large_2"}, ...)

or

@idlarge = ($Data_Hash{Call_Hash}->{"TST_" . "IDS_Large_1"}, $Data_Hash{Call_Hash}{"TST_" . "IDS_Large_2"}, ...)

since consecutive indices contain an implicit deference.

Update: I misunderstood the spec. Assuming that your access methods are correct, you actually want two completely separate hashes. The values of %Call_Hash are being used as keys in the %Data_Hash. The data structures you want are given by:

%Data_Hash = (
    OPT_VGS_1 =>0,
    OPT_VGS_2 =>0.02,
);
%Call_Hash = (
    TST_IDS_Large_1 =>2.9E-12,
    TST_IDS_Large_2 =>1.85E-12,

    TST_IDS_Small_1 =>1.86E-9,
    TST_IDS_Small_2 =>2.83E-9,
);
[download]

except that the values of %Call_Hash should correspond to keys in %Data_Hash.