That ain't no simple string. That are HTML, and HTML ain't so simple. In fact, if you have anything else you want to do for the rest of the week, don't bother trying to write code to parse HTML. Use something like HTML::TreeBuilder instead.

However, even before that you should take a regex refresher course. I'd recommend that you start with perlretut, perlrequick, perlre and perlreref.

In the mean time $intext =~ s/$orgtext/$newtext/ms; probably wants a /g to replace all the strings.