my $tbltext = qq(<tr><td>a1</td><td>a2</td><td>a3</td><td>a4</td></tr><tr><td>b1</td><td>b2</td><td>b3</td><td>b4</td></tr><tr><td>c1</td><td>c2</td><td>c3</td><td>c4</td></tr>);

sub fix_column {
    my $s   = shift;
    my $nth = shift;
    my $counter = 0;
    $s =~ s/(<td.*?\/td>)/
            ++$counter ==  $nth #is this the nth cell?
            ? "${1}newtext" #if yes, add "newtext"
            : $1 #otherwise, leave it
            /ge;
    return $s;
}

$tbltext =~ s/(<tr.*?>)(.*?)(<\/tr>)/$1.fix_column($2, 3).$3/ge;

print "$tbltext\n";
__END__
<tr><td>a1</td><td>a2</td><td>a3</td>newtext<td>a4</td></tr><tr><td>b1</td><td>b2</td><td>b3</td>newtext<td>b4</td></tr><tr><td>c1</td><td>c2</td><td>c3</td>newtext<td>c4</td></tr>