Code below, press the validate button to see the problem with line wrap and 'newline chars' as per my original post.

#!/usr/bin/perl

#
#Text widget word wrap problem demo
#
use strict;
use Tk;
use Tk::TFrame;

my $mw = MainWindow->new;

my $big_font = $mw->fontCreate(
          -size => 11, -family => 'Terminal',
          -weight => 'bold');

$mw->title("Demo App");

my $seg1_f = $mw->TFrame(-label => 'Segment 1', -relief => 'groove', -
+borderwidth => 2);

my $ts_box  = $seg1_f->Text(-wrap => 'word', -width => 60, -height => 
+6,
                                        -font => $big_font);
$ts_box->grid( -padx => 4, -pady => 4, -row => 0, -column => 4,
                                        -sticky => "nsew", -rowspan =>
+ 4);
$ts_box->menu(undef);
$ts_box->tagConfigure("badc", -foreground => "red", -background => 'ye
+llow');

my $val   = $seg1_f->Button(-text => "Validate", -command => [\&valida
+te_text, 4]);
$val->grid(-row => 0, -column => 3, -padx => 4, -pady => 5);

$seg1_f->grid(-row => 0, -column => 0, -sticky => "nsew",-padx => 2, -
+pady => 2);
$mw->resizable(0,0);

$ts_box->insert('end', "This is some test text with some invalid @ cha
+racters");
$ts_box->insert('end', " If you click the Validate button the bad char
+s &should be");
$ts_box->insert('end', " highlighted! Note that a newline character");
$ts_box->insert('end', "\n hightlights to the end of the line not just
+ the single");
$ts_box->insert('end', " character position. Is this a bug?");

MainLoop;

# Validate the text field
sub validate_text{
my $i_st;
my $idx;
my $cnt;
my $ch;
    $ts_box->tagRemove('badc', '1.0', 'end');
    my $wstring = $ts_box->get("1.0", "end");
    $idx = length($wstring)-1;
    if ($idx == 0) {return(0)}
    $cnt = 0;
    while ($idx != $cnt) {
        $ch = substr($wstring, $cnt, 1);
        if (!($ch =~ /[A-Za-z0-9 \/.,=?*+]/)) {
            $i_st = sprintf("1.%d", $cnt);            # bad char
            $ts_box->tagAdd('badc', $i_st);
        }
        $cnt++;
    }
}
[download]

The behaviour doesn't need a Tk::TFrame, it stays the same using Tk::Frame, too.

Moreover, it seems nothing is highlighted if the line ending in the newline has the maximum length, which would concern me much more. Maybe indicate the newlines by something (a pilcrow ¶ maybe?).

map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]

I tried replacing the newlines, but I'm having a problem, it doesn't do what I expect. Because I want the replace to appear in the text box I tried:

$ts_box->FindAndReplaceAll(-regexp, -nocase, "[\n]", "%");
[download]

which appeared to have no effect, so as a sanity check I tried:

$ts_box->FindAndReplaceAll(-regexp, -nocase, "[+]", "%");
[download]

which works and replaces all the '+' chars with '%' chars. What am I doing wrong to stop \n matching?

Also note that nothing after a newline gets validated.
map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]