Re^2: sorting and merging in perl

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A1,B1,ACT_DATE,INACT_DATE
7900724655,200906888,20180416,20180522
7900724655,200906889,20180601,20180720
7900724655,200906889,20180724,20180728

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7900724655,200906888,20180730,20180830
7900724655,200906890,20180905,20180930
7900724655,200906890,20181005,20181030
7900724655,200906890,20181104,

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7900724666,200906868,20180416,20180522
7900724666,200906869,20180601,20180720
...
7900724666,200906890,20180905,20180930
7900724666,200906890,20181005,20181030
7900724666,200906890,20181104,

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7900724655,200906889,20180601,20180728
7900724655,200906890,20180905,
7900724666,200906869,20180601,20180728
7900724666,200906890,20180905,

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