First you associate the extension to a type and then the type to executable(s). Look inside the "HKEY_CLASSES_ROOT\pl_auto_file".

And while you are at it, see whether you have a HKEY_CLASSES_ROOT\Perl key. Maybe just the mapping from the extension to the type was broken, 'cause "ext_auto_file" is something you end up with if you doubleclick an unknown extension and select a program to run it with.