# pass  1   2   3
                                       #      --- --- ---
sub test {
   my @matches;
   '' =~ /
      (?{ push @matches, 'a' })
      (?{ print(scalar(@matches), "\n") })   # 1   2   3
   /xg;  
   print(scalar(@matches), "\n");            # 1   0   0
}

test() for 1..3;