The /i is a modifier at the end of a regex, just like /g. Your code to modify $body_temp has /i in the right place.

If you were to use code like I provided in Re: Removing duplicate line, just add i after g in the second substitution. (I would recommend using my code, mostly because I'm full of True Perl Pride today.) We'll say something like:

s/(a href=.+?\.)html/$1asp/gi;

The order of the modifier doesn't matter in this case. (I can't think of a case where it does.) The important thing is that it's never /g/i or /i/g. Just combine them after the final slash of the regex.

You can learn more about regular expressions and their modifiers in perldoc perlre.