But note the following, from perl5140delta:

?PATTERN? (without the initial m) has been deprecated and now produces a warning. This is to allow future use of ? in new operators. The match-once functionality is still available as m?PATTERN?.

So with v5.14.4, I get:

22:39 >perl -le "$_ = qq[abc]; print 1 if ?b?;"
Use of ?PATTERN? without explicit operator is deprecated at -e line 1.
1

22:39 >
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Hope that helps,