#!/usr/bin/perl -w 
# Caveat:  untested code. Don't do anything foolish with it. 

use strict;
use Tie::File; 

tie  my @ry,"Tie::File", "MyFile.csv" 
or die "MyFile.csv:$!";

my $i = 10;  # or whatever 
my @j = split(",",$ry[$i]);
$j[1] = "This is a fence";
$ry[$i] = join(",",@j);

#
#############################