#!/usr/bin/env perl

use strict;
use warnings;

use Tie::File;

my ($file, $index, $replacement) = @ARGV;

tie my @records, 'Tie::File', $file or die $!;

for (@records) {
    my @fields = split /,/;
    $fields[$index] = $replacement;
    $_ = join ',' => @fields;
} 

untie @records;

##</code><code>##

$ cat fred
A,B,C
D,E,F
G,H,I

$ pm_tie_file_split_fields.pl fred 2 Z

$ cat fred
A,B,Z
D,E,Z
G,H,Z