I believe you have to predeclare the variable as VT_BYREF first. Something like:

my $Status;
my $Is_In = Variant(VT_BYREF | VT_VARIANT, 0);
while (1) {
    $Status = $Rdr->IsCardIn($Is_In);
    print "Is Card In:$Status ; IsCardIn:$Is_In\n";
    sleep(1);
}
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Look at the "Variants by reference" section of the Win32::OLE::Variant docs.