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


##</code><code>##

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