#this while loop isn't right, but I don't know what is...;
while ($tbltext =~ /<tr.*?>/cg) {
    $counter = 1; #reset the counter; 
    $tbltext =~ s/(<td.*?\/td>)/
                   ++$counter ==  $nth #is this the nth cell?
                   ? "${1}newtext" #if yes, add "newtext"
                   : $1 #otherwise, leave it
                   /ge;
    $tbltext =~ /<\/tr>/cg; #find the end of the row
} #exit the loop