The problem might where you are setting the background brush for the menu. You need to pass the handle of the brush, rather than the brush itself. So instead of this:

my $mi = pack( 'LLLILLL',
    12 * 4,
    MIM_BACKGROUND,
    0,
    0,
    $po,               # <=== problem might be here
    0,
    0,
);
[download]

you need to do this:

my $mi = pack( 'LLLILLL',
    12 * 4,
    MIM_BACKGROUND,
    0,
    0,
    $po->{-handle},    # <=== change to this
    0,
    0,
);
[download]