You can experiment with the following code. It might need some tweaking on MSWin or Mac:

#!/usr/bin/perl
use warnings;
use strict;

use Tk qw{ MainLoop Ev };

my $mw = 'MainWindow'->new;

$mw->Label(-text => 'Press any key')->pack;
$mw->Entry(-textvariable => \ my $text, -state => 'readonly')->pack;
$mw->bind('<KeyPress>', [sub { $text = $_[1] }, Ev('K')]);

my @mods = ("", qw( Alt Shift Control Mod4 ));

for my $p1 (0 .. $#mods) {
    for my $p2 (0 .. $#mods) {
        next if $p2 == 0 && $p1 != 0
             || $p2 && $p1 >= $p2;

        for my $p3 (1 .. $#mods) {
            next if $p3 <= $p2;

            my $prefix = join '-', grep length, @mods[$p1, $p2, $p3];
            $mw->bind("<$prefix-KeyPress>", [sub { $text = "$_[1] + $_
+[2]" },
                                             "$prefix", Ev('K')]);
        }
    }
}
$mw->bind("<Alt-Shift-Control-Mod4-KeyPress>",
          [sub { $text = "$_[1] + $_[2]" },
           'Alt-Shift-Control-Meta', Ev('K')]);

MainLoop();
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