For converting Perch blog posts into Kirby text files
Firstly, convert.php will run the process and it goes in perch/templates/blog/pages. Modify the settings in this file to your own requirements and make sure that the perch_blog_custom function in the script is appropriate for your site. Then, you will need to create a page on your site that uses this as the master page. Then, open the page in your browser. The script will generate the blog posts in a folder called /convertedposts in the root of your site as long as the following template is in place.
Secondly, the process uses _convert-posts.html as the Perch template to create the Kirby text files in /convertedposts. In my case, I have _convert-posts.html in perch/templates/blog/_hidden but you can put it wherever you want. Just make sure that _convert-posts.html is in the correct location before you run convert.php.
With big thanks to Nils Mielke who wrote the initial script that I have adapted for my own use.