There is certainly always the potential for trouble when upgrading. As you point out lots of modules will typically be upgraded in the process too.

The last time I made a major change, I went from Perl 5.6 to Perl 5.10 (I skipped 5.8). And yes some stuff "broke". As I remember, some of this had to do with deprecated syntax of some code. It would work, but there would be warnings. So I had to make modifications to get things running "clean". In a few cases I had trouble installing a more up to date module and had to do some futzing around.

I'm an ActiveState user and I waited awhile after 5.10 became available so that I could get all the modules that I needed. If you are on Unix, that may not be an issue for you. My next adventure will probably be to just 5.12 as Active State isn't fully on-board with 5.14 yet.

You didn't mention if you were also upgrading from 32 to 64 bit. I don't know the pitfalls in that. But if you are doing that, its probably worth a separate question.

Overall the impact of upgrading was well worth it. The performance of some things that I care greatly about, like sort is vastly better in Perl 5.10 vs 5.6. And I've found that backward compatibility is very good.