possibly this was a recent change?

Relatively recent. The main change was this one in Perl 5.26.0, released in May of 2017, with some preliminary changes (these) in 5.24.1, released in January of 2017.

There are many .pl scripts and I'd rather avoid having to manually change them all. Is there a way I can go to accomplish that?

What change are you talking about - the include path? One way to add paths to @INC globally is by setting the PERL5LIB environment variable (taking care not to clobber its contents if it's already set).

I'm only going to use this local site to test new code or other things before I implement them on the unix site.

I've worked with Perl on both Windows and *NIX for many years, and let me just say that things are different on both OSes - not necessarily better or worse, but certainly different. Sure, getting Perl code to run on both is possible, but in this situation, it sounds like you've got a test/development server, which you want to set up on Windows, and a production server, which is *NIX. I do have to say that I think you'd be much happier if your test/dev system was *NIX as well, as things will mesh so much better that way - no fiddling with file permissions every time you sync the two, no differences in paths between the two, and so on. You might want to consider setting up a Linux virtual machine on your Windows system...