$\ = "\n";
$, = ",";
print 'a string' =~ m/(.(?{pos()+=4}))/g;
__END__
prints "a, ,s,t,r,i,n,g". It should print "a,i"

##</code><code>##

'14567890ab' =~
  m/(.(?{$char = substr($_,pos(),1);
         print "> ".pos()." \"$char\" ";
         if ($char eq '4' or
             $char eq '6' or
             $char eq '8')
           {
             print "+".(0+$char)." ";
             pos() += 0+$char;
           }
         else
           {
             print '+0 ';
           }
         printf "< %2d \"". substr($_,pos(),1)."\" ", pos();
         })

      (?{printf "-> %2d\n", pos}))+/x;
__DATA__
> 1 "4" +4 <  5 "8" ->  1 # alter pos()
> 2 "5" +0 <  2 "5" ->  2
> 3 "6" +6 <  9 "b" ->  3 # alter pos()
> 4 "7" +0 <  4 "7" ->  4
> 5 "8" +8 < 10 "" ->  5  # alter pos()
> 6 "9" +0 <  6 "9" ->  6
> 7 "0" +0 <  7 "0" ->  7
> 8 "a" +0 <  8 "a" ->  8
> 9 "b" +0 <  9 "b" ->  9
> 10 "" +0 < 10 "" -> 10

##</code><code>##

__SIG__
use B;
printf "You are here %08x\n", unpack "L!", unpack "P4", pack
  "L!", B::svref_2object(sub{})->OUTSIDE;