You have to save the additional field to the datafile. Todo so, open Post.pl and look for
print FILE qq~$subject|$name|$email|$date|$username|$icon|0|$user_ip|$message|$ns|\n~;
and reaplce that with
print FILE eld\n~;
then look for
print FILE qq~$subject|$name|$email|$date|$username|$icon|0|$user_ip|$message|$ns|\n~;
and replace that with
print FILE eld\n~;
now in Display.pl, you have to replace
($msub, $mname, $memail, $mdate, $musername, $micon, $mattach, $mip, $postmessage, $ns, $mlm, $mlmb) = split(/[\|]/, $_);
with
($msub, $mname, $memail, $mdate, $musername, $micon, $mattach, $mip, $postmessage, $ns, $mlm, $mlmb, $testfield) = split(/[\|]/, $_);
Now, you have the variable $testfield available the Display sub so that you can simple add it to the post output.
But there is something you have to change too: if you edit your post, you'll loose this extra field in the datafile. To prevent this, you have to edit ModifyMessage.pl too:
($msub, $mname, $memail, $mdate, $musername, $micon, $mattach, $mip, $mmessage, $mns, $mlm, $mlmb) = split( /\|/, $messages[$postid]);
replaced by
($msub, $mname, $memail, $mdate, $musername, $micon, $mattach, $mip, $mmessage, $mns, $mlm, $mlmb, $testfield) = split( /\|/, $messages[$postid]);
and
$messages[$postid] = |$username\n~;
must be replaced by
$messages[$postid] = |$username|$testfield\n~;
I didn't test this code and there are hundreds of other places where this additional code should be added like in the preview and stuff but I don't have the time to write you a whole mod out if this here right now.