You wont see /g being support by qr//'s. I think i can say that as a definitive statement. Part of the reason is that with the exception of the /o modifier then only modifiers allowed on a qr// are those that can apply to a subsection of a pattern. You have to ask yourself what would happen in a case like:

my $qr_g=qr/foo/g;
my $other=qr/balh $qr_g/;
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What would it mean to have half of a pattern be /g and the other half not? What sense would it make if somebody used $other specifically without /g?

Also, the /g modifier is a property of the PMOP that will be executing a pattern like operator, whereas msix are properites of the resulting compiled pattern. In short they arent even stored in the same place.

Forget the idea of qr//g, its not going to happen.