If it are old perl scripts (rather than modules written with a full and complete test suite), the only way to make sure they run, is by trying to run them.

Set up a modern Perl system on a separate PC, mimic your production environment on that machine but make sure your scripts cannot access the real production environment (e.g. mission critical databases and such) and start the scripts. Feed it the data it need and if the scripts run to completion without dieing, check its output (messages, files, changes to the databases, ...).

If all of that checks out fine, you can be reasonably certain that upgrading Perl did not cause major problems.