I used to use "our" and 3-arg open() until I started working in a shop that supports all kinds of ancient machines (Redhat 6.2, with Perl 5.005_03) because that's what a particular customer bought way back when and they don't want to upgrade.

So now I just avoid all the new-fangled constructs (which don't really add much anyway) and try to avoid modules that don't work on 5.005. Sometimes I have to go download an older version of a module, such is life.

In some cases I find that it makes sense to minimize the dependency on external modules and do as much as possible with Perl core. It's pretty powerful as it is... For instance, why use Switch.pm when the Perl Cookbook has a perfectly good, portable example of a switch statement that's made up entirely of built-ins and works even on ancient versions of Perl? (Switch.pm wasn't a standard module back then either).