$window->TrackPopupMenu(
    $menu->{mymenu},
    20, 20,
    0, 0, 0, 0,
    TPM_LEFTALIGN | TPM_TOPALIGN | TPM_LEFTBUTTON, 
    \&callback
);

sub callback {
    my ($self, $message, $wParam, $lParam) = @_;
    # Process message here...
    return 1;
}