#!/usr/bin/perl
use strict;
use warnings;
my %fh;
while ( <DATA> ) {
    my @field = split /,/;
    my $file  = splice @field, 2, 1;
    if ( ! $fh{$file} ) {
        open ($fh{$file}, '>>', $file) or die $!;
    }
    print { $fh{$file} } join ',', @field;
}
__DATA__
1,2,foo,3
4,5,bar,6
7,8,foo,9