There is a bug in the program that makes it

• handle the last column badly as key column if it has newlines at the end
• not print the header at all

I've tested the file against this data:

field1,field2,field3,split
zweiradlinss,89803607,330525685618,3
zweiradlinss,89803607,310286767428,77
[download]

with this command line:

perl -w part.pl part.csv --header-line=1 --column=3 "--separator=,"
[download]

Thanks for the report! I've fixed the top version and updated it to v0.06.