It's not evident that that is a "nice use if (X)*" given that different versions of Perl handle the rather bogus expression differently. The same result can be achieved using zero width assertions:

"babbbbbcbbbcx" =~ /(?:b)(?!.*b)(..)/;
print ">$1<\n"; # >cx<
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Update s/\Q(?!(?=.*b))\E/(?!.*b)/. Thanks to ikegami for pointing out the redundancy.