my $new = '';
for (;;) {                       # or: for ($orig)
   if (/\G(.*?)(this)/sgc) {
       say "Matched $2 at $-[2]";
       $new .= $1 . 'that';
       redo;
   } else {
       /\G(.*)/sgc;
       $new .= substr($_, pos);
       last;
   }
}