Is it possible to have a capture for the purposes of backreferences, but not returned as part of the match?

# Example
@x = 'bogus firstblahjokeblahthird bogus' =~ /(first)(?:blah).*?\2(thi
+rd)/; # doesn't work
# @x is empty
[download]

(In fact, I suspect the above doesn't work because, while there is eventually a 2nd capture group, it hasn't captured anything before the backreference. But that isn't really the point here.)

I don't really have a specific need, but it just seems interesting to say "this capture is only for backreference, and not for returning as a capture".