If you just need to lowercase one letter, this: $input =~ s/\.(.)\.WI$/_\L$1_w/;will do the trick. The \L modifier in the result side renders the next letter in lowercase. It's not that obscure; just look in perlman:perlre.

If you want to do more fancy processing, you can use the </code>/e</code> modifier. The same page has details.

HTH