$string =~ s/(\Q$first\E).*?(\Q$last\E)/$1$reserve$2/;

##</code><code>##

$string =~ s/(?<=\Q$first\E).*?(?=\Q$last\E)/$reserve/;

##</code><code>##

$first_start = index($_, $first);
if ($first_start >= 0) {
   $first_end = $first_start + length($first);
   $last_start = index($_, $last, $first_end);
   if ($last_start >= 0) {
      substr($_, $first_end, $last_start, $replace);
   }
}