#!/usr/bin/perl

use strict;
use warnings;

my $string = "<td>foo</td><td>foo</td><td>foo</td><td>foo</td><td>foo</td><td>foo</td>";
my $col    = 5;
my $link_anfang = '<a href="http://board.perl-community.de">';

my $min    = $col - 1;

$string =~ s!((?:<td>.*?</td>){$min})<td>(.*?)</td>!$1<td>$link_anfang$2</a></td>!;

print $string;