Assignment of UnManagedStruct in Perl6

I'm writing an SQLite extension. I have it working fine in PIR, but the
bridge to Perl 6 is causing problems. Specifically, this Perl 6 code works:

SQLite::pmc_check(SQLite::open("test.db"));
# Returning 0x2d1efa0
# PMC 0xedf1a8 data pointer 0x2d1efa0
# PMC 0xedf1a8 data pointer 0x2d1efa0

But this code does not:

our $db; SQLite::pmc_check($db = SQLite::open("test.db"));
# Returning 0x2d1a850
# PMC 0x263f504 data pointer 0x2d1a850
# PMC 0x263f590 data pointer 0x0

SQLite::open is implemented in PIR, as follows:

.sub open
    .local pmc x
    x = open_wrapper("test.db")
    pmc_check(x)
    .return(x)
.end

pmc_check is a simple C function to tell me what's going on inside my
UnManagedStruct:

void pmc_check(PMC* pmc) {
    printf("PMC %p data pointer %p\n", pmc, pmc->data);
}

The difference is, of course, that it goes through a Perl 6 variable
assignment.

From this I can conclude that my PIR open() code correctly returns an
UnManagedStruct PMC; however, when Perl 6's assignment operator assigns
this to a variable, a new, empty UnManagedStruct PMC is created and no
longer contains the right data.

I have no idea how to fix this, but it looks like the assignment
operator is broken.

Simon