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

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

sub moritz {
  my ($s1, $s2) = @_;
...
    substr( $$s1, $pos, 1 ) = substr( $$s2, $pos, 1 );
  }
}