I just pulled a PHP site out of my archive that I had taken down 7 years ago. Only one function call (each) had been removed, and everything else still works. There's been some major changes in some parts of PHP though; if I had been following along, I'd have gotten a deprecation warning, but I jumped two or three major versions.
I'd be pretty confident in any Perl code written for 5.8 or newer to continue working, with CPAN libraries as well. Earlier code is probably ok as long as you don't deal with binary data or non-ascii characters; 5.8 was the unicode reckoning.
If you ran FreeBSD 4, most of FreeBSD 13 will feel familiar. Although if you had a kernel driver for FreeBSD 4, it will likely need a lot of help to work in FreeBSD 13.
Of course, there are other communities where everything is thrown away every 6 months. I don't have that kind of time.