You can't just retain the necessary modules... Major versions of perl are all binary incompatible. If these modules rely on XS, thus have a portion in compiled C, they'll have to be reinstalled for the new version of Perl.

As for issues... The major issue I can think of is related to UTF8. Indeed from 5.6.0 on, perl has native UTF-8 support. Sometimes perl will convert between what it thinks is ISO-Latin-1 and UTF-8, without you asking for it, or even where it is not what you want. It can, and will, happen when you concatenate normal strings, entered in Perl, with the output of XML::Parser.

You'll just have to test it. Make a new installation of perl, retaining the old one, next make a copy of your script that'll run under the new perl, and test it. Test it, test it, test it! All while keeping the old copy in place, of course, and making sure it'll chug along as before.