If the OP wants to capture key stroke, then should use KeyRelease etc. If what the OP cares is whether the context has been modified, then:

use Tk;
use Tk::Dialog;

use warnings;
use strict;

my $MW = MainWindow->new(-title => "Tk::Text test",
                         -width => 200,
                         -height => 200);

my $text = $MW->Text(-height => 10,
                     -width  => 40,
                     -wrap   => 'word');
$text->pack(-side => 'top', -fill => 'both');
$text->bind( '<FocusOut>' => \&callback);
my $text2 = $MW->Text(-height => 10,
                     -width  => 40,
                     -wrap   => 'word');
$text2->pack(-side => 'top', -fill => 'both');

MainLoop;

sub callback {
    if ($text->editModified()) {
        $text->Dialog(-title=>"Modified",-text=>"\$text has been modif
+ied")->Show();#or whatever you want
        $text->editModified(0);
    }
}
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