perlexperiment: mark regexp backtracking verbs as accepted
perlexperiment: (?{}) and (??{}) are not experimental

...but we need some more explanation of its limitations.  This text
was provided by Yves Orton on perl5-porters in message
<CANgJU+UXO7tKZgOvbwufFxAjupOcKVPdDBNkRrT7DWKdv9tBgw@mail.gmail.com>
perlexperiment: mark lvalue subroutines accepted