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