use Devel::Peek;
$str = 'a string';
print 'x' x 50, "\n";
Dump($str);
$str =~ m/(?:.(?{Dump($_);Dump(pos());pos()+=3;
                 Dump($_);Dump(pos())}))/g;
print STDERR "OUT\n";
Dump($str);

##</code><code>##

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