This code illustrates that the op only performs the match once:

my $type;
my $r = qr/(?{print "$type\n"})/;
for (1..3) {
  $type = 'Regular';
  /$r/;
  $type = 'Once only';
  ?$r?;
}
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On the broader subject of modifying the optree...well, it hasn't really occurred to me very often. The most common reason to do it is to introduce new syntax, which I guess is what you're suggesting, after a fashion. If I were up to it, I would probably want to implement a new operator.

(update: But there's more to adding syntax than modifying the optree. It's gotta parse, too.)