Just off the top of my head ( or, depending on how you look at it, pulled out of my a**) :-) , here is a snippet I found laying around, that might give you ideas. You could setup the callback to detect the key, then insert with tags, assigned from a hash or something. I havn't tested it with tags and what Undo will do with it.

#!/usr/bin/perl
use strict;
use warnings;
use Tk;
use Tk::TextUndo;

my $top = MainWindow->new;

my $text = $top->TextUndo->pack(
    -expand => 'yes',
    -fill   => 'both'
);

$text->bindtags( [ $text, ref($text), $text->toplevel, 'all' ] );

$text->bind( '<KeyPress>', [ \&callback, Ev('K') ] );

MainLoop;

sub callback {
    my ( $widget, $keysym ) = @_;

    print "\n$keysym key pressed";

    return
      if (
        (
                length($keysym) > 1
            and $keysym =~ /\p{Upper}/
            and $keysym !~ /Delete|BackSpace|Return|Tab/
        )
        or (    $keysym eq 'Delete'
            and $widget->index('insert') eq $widget->index('end -1c') 
+)
        or (    $keysym eq 'BackSpace'
            and $widget->index('insert') eq '1.0' )
      );

    print ' - text is edited.';
}
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