#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11117224
use warnings;

my $match = qr[([ab]+)([ab]+)];
my $str = 'aba';

$str =~ /^ $match $ (?{  print "1: $1-$2\n" }) [c] /x;
$str =~ /^ $match $ (?{  print "2: $1-$2\n" }) (?!) /x;
$str =~ /^ $match $ (??{ print "3: $1-$2\n"; qr[(?!)] }) /x;

# The fact that this is a defined pattern probably means
# there is a lesser chance of it being optimized away.

$str =~ /^ $match $ (?{  print "4: $1-$2\n" }) (*FAIL) /x;