Maybe I am wrong, but contrary to the others who have replied already, I think you are misinterpreting your problem (sorry if I misinterpreted you! :).

Do you want the following:

%hash = (
         g1 => [value1, value2, value3],
         g2 => [value4, value5, value6...]
);
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i.e. do you want each key to have multiple values associated? If this is the case you need a reference to an array containing all the values (see perlref). You already have that situation in the example you give: g1 => [$start_date1,$end_date1], so, to concatenate more values, you will need a reference to an AoA (array of arrays). Again, sorry if I totally misinterpreted your problem, but if you think I am close, let me know and we will elaborate it a little more