Hello, QM,

Haven't heard about such capture groups for only backreferences.
Interestingly, if any simple overcome exist? This overcome below looks ugly:

#!/usr/bin/perl -wl

print map "[$_]", "bogus firstblahj?keblahthird bogus" =~ /
    (?| (*F)
        | first ( (blah) .*? \2 ) third (?{ $L = $1 }) (*THEN) (*F)
        #       1 2    2        1
        | (??{ defined $L ? "" : "(*F)" })
          (first) (??{ print "mid-match:[$L]" ; quotemeta $L }) (third
+)
        # 1     1                                               2     
+2
        )
    /x;
[download]

output:

mid-match:[blahj?keblah]
[first][third]
[download]

Idea here is to use conditional '(?|...)' with 2 groups, save mid-match (if full match is successful), fail first branch anyway, then match same thing on alternative branch rewriting $1 and $2, and using previously defined mid-match to avoid captures.