I see. So the info that Tcl::Ev() should come first can be found as example in http://search.cpan.org/grep?cpanid=VKON&release=Tcl-Tk-1.04&string=bind&i=1&n=1&C=9, https://metacpan.org/source/VKON/Tcl-Tk-1.04/tk-demos/widget

$T->tagBind(qw/demo <Motion>/ => [sub {
    my($x, $y) = (shift,shift);
    my($text, $sv) = @_;
    #my $e = $text->XEvent;
    #my($x, $y) = ($e->x, $e->y);
    my $new_line = $text->index("\@$x,$y linestart");
    if ($new_line ne $last_line) {
        $text->tagRemove(qw/hot 1.0 end/);
        $last_line = $new_line;
        $text->tagAdd('hot', $last_line, "$last_line lineend");
    }
    show_stat $sv, $text, $text->index('current');
    }, Tcl::Ev('%x','%y'), \$STATUS_VAR]
);
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It is also documented in Tcl

4. As a special case, there is a mechanism to deal with Tk's special event variables (they are mentioned as '%x', '%y' and so on throughout Tcl). When creating a subroutine reference that uses such variables, you must declare the desired variables using Tcl::Ev as the first argument to the subroutine.