/i goes at the end of the regex, eg. $text =~ s/foo/bar/i;

I'd recommend you look at perlop for a description of how to use the various regexp operators in Perl, as well as perlre for an in-depth discussion on how to use regexps.

Those who know that they are profound strive for clarity. Those who
would like to seem profound to the crowd strive for obscurity.
            --Friedrich Nietzsche