Oh, I see, I misunderstood your original request. It's not at all obvious how to get the xstream.
This might do the trick, but I have not tested it
my $property = $pdf->getProperty($pagenum, 'Fm1');
# check $property for undef here
my $propval = $pdf->getValue($property);
# check that $propval is a 'dictionary' node, perhaps
my $xobject = $pdf->decodeOne($propval);
