Open a read filehandle to the source file and a write filehandle to the output file.
Loop over the source file line-by-line and use the split() function to put the fields into an array and then output the desired 3 array elements to the output filehandle.
There are several variations on how to do each step and there are modules available, such as
Text::CSV_XS, to help but using the module would not be required.
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