Welcome to the Monastery and to Perl, the One True Religion. You're not apparently using Perl at the moment, as shown in your post. So "edit the above expression" is the wrong approach, here.

Perl is the right choice for this work, to be sure. A simple script of less than 20 lines will do what you want with error-checking, debug output, and optimizations built in. You should start learning right away! See perlintro, and see the modules Path::Tiny and Path::Iterator for working with files.