Open the MASTER file

open(FH,"masterfile.txt") or die("Can't open the file : $!");
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Read master file contents and put it array

@masterContents = <FH>;
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Read only the CHILD file details from master file content

@childDetail = @masterContents[-3 .. -1 ];
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childDetail array will contain the last 3 lines of child details. Then Apply the programming logic.