The rule has generally been that you can take a 20 year old Perl script and it will still just run. This has its own complications though, as it makes it harder to advance the language, and then people feel it's being left behind (the Perl 6 stuff came after that feeling developed).
It's a longstanding problem in language design and communities, stability vs advancement.
Also, if you desperately need an older version, you can usually install that older version rather than the newest.
Anyways, there's things you can do. I still work on code I started in my dorm room (in the 90's). Cranky CPAN modules aren't too big of a problem.
Core modules are all I would worry about, but I'm pretty sure those are held to the same standard as far as backwards compatibility.
One or two modules might have been ejected from core or moved into core, but those should be easily found on CPAN as well.
In the end, getting something working from scratch on a newer Perl that has a lot of CPAN dependencies (as opposed to just updating Perl) might be a little annoying in that you have to track down all the module versions, but it's far from impossible, or even all that hard.
For example, the CGI module, which was a core module but historically seen as fairly bad (at least as of now, and at least in some of its uses, it supported a wide set of use cases), it was removed from core and housed on CPAN. If you follow the link provided and check the past versions (dropdown as part of the module path and name), you'll see there are many versions shown, possibly every public version, and going back to 1998 for CPAN, and 1995 for 1995 for the BackPAN. Each of those is visible as an item with it's documentation at the time and the module available to download and use. You can also access the CPAN testing matrix, and if you dig around you can actually find the results for some tests back in 1999.
If I was responsible for getting some old Perl app to run on a more modern system, I would by much more worried about the OS changing in a complex way than I would about getting the Perl code to run again as expected. Which is to say, I wouldn't be worried much at all.
I usually use Python these days when dealing with SaaS apps since the ecosystem is much better, and I want to learn it anyway.