Hmm, validating only makes sense for single-line widgets ... anywho

#!/usr/bin/perl --
use Tk;
use strict;
use warnings;
Main(@ARGV);
exit(0);
sub Main {
    our $status = "";
    my $mw = tkinit;
    my $t = $mw->Scrolled(qw/
        Text
        -relief sunken
        -borderwidth 2
        -setgrid true
        -height 30
        -scrollbars e
    /)->pack(qw/
        -expand yes
        -fill both
    /);
    my $l = $mw->Label(
        -textvariable => \$status,
        -relief => 'groove',
        -font => "Arial",
        -anchor => 'center',
    )->pack(
        -side => 'bottom',
        -anchor => 's',
        -fill => 'both',
    );
    use DDS; Dump( $l );
    $t->bind( '<<Modified>>' => [ \&onModified , $t , $l ]);
    MainLoop;
}
sub onModified {
## warn " @_ "; ## go-figure , probably a bug
##  Tk::Text=HASH(0xeb324c) Tk::Frame=HASH(0xed2fcc) Tk::Label=HASH(0x
+fbfb94)
    my( $t,  undef, $l ) = @_; 
    if( $t->editModified ){
        $t->editModified(0); ## reset count
        if( "invalid" ){
            $t->bell;
            $::status = "invalid, forever";
            $l->configure ( -background => 'red' );
        }
    }
    return;
}
[download]

widget