sub moritz {
  my ($s1, $s2) = @_;

  my $pos = 0;
  while ( 0 < ( $pos = index $$s1, "\000", $pos ) ) {
    substr( $$s1, $pos, 1 ) = substr( $$s2, $pos, 1 );
  }
}

my $s1="\0x\0z";
my $s2="ABCD";

moritz(\$s1,\$s2);
print $s1 ne 'AxCz' && 'not ','ok';

##</code><code>##

sub moritz {
  my ($s1, $s2) = @_;

  my $pos = 0;
  while ( -1 < ( $pos = index $$s1, "\000", $pos ) ) {
    substr( $$s1, $pos, 1 ) = substr( $$s2, $pos, 1 );
  }
}