BEGIN {
    require Tk;
    import Tk;
}
$top = tkinit;
$b = Checkbutton::new($top, "-text"     => "click button 3 for value",
                            "-variable" => $foo,
                            "-onvalue"  => "yep",
                            "-offvalue" => "nope");
tkbind($b, "<3>", "::printvalue");
tkpack $b;
tkmainloop;

sub printvalue {
    print "checkbutton value is $foo\n";
}