If you really want shorter code then: 
use warnings;
use strict;

/^([^,]+)/ && open F, ">>$1" and print F while <>;

__END__

[download]
:-) The way it works is that it opens a file in append mode for every line that matches the regular expression using the first field as the file name and prints the current line to the end of that file.

In reply to Re^3: Split up file depending unique values 1st column by jwkrahn
in thread Split up file depending unique values 1st column by GertMT

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