This module helps "bump" the version of the language used in a piece of Perl code to a more recent version.

The Perl of 2024 (v5.40) is not the Perl of 2002 (v5.8). It's a much improved language, with many new and useful features for large code bases. So, why not start using it now?

Why even bother declaring a language version in your code?

You might have heard that Perl has amazing support for backwards compatibility. You may think it's because the language is stagnating. It's actually the opposite! It's just that accessing the new features is a bit more complicated than just using the latest perl binary.

From the feature module documentation:

It is usually impossible to add new syntax to Perl without breaking some existing programs. This pragma provides a way to minimize that risk. New syntactic constructs can be enabled by use feature 'foo', and will be parsed only when the appropriate feature pragma is in scope.

Yes, you read that right. Inside your own files, on a file-by-file basis, you get to decide which version of Perl you get.

Behind its simple looks, use VERSION actually enables Perl's amazing support for backwards compatibility, and also fuels Perl's ability to continue to bring in new and sometimes backwards-incompatible features. It does that by loading "version bundles", which enable (and sometimes disable) features that were added to the language over time.

When Perl starts, it reads your code from line 0. At line 0, Perl supports most of the language features of Perl 5.8 (published in 2002). Except for some hard deprecations (Perl has sometimes moved forward somewhat abruptly), any sensibly-written code from twenty or more years ago will just work.

Over time, as new features were added to Perl, the ones that broke backwards compatibility, or that were a bit too experimental to be made part of the language proper were put behind feature guards. You have to opt-in to them. This is what use VERSION does: it signals that the code you wrote was against the given VERSION of the language.

If the first line of your code is a use VERSION line, you get a more useful Perl, starting from line 1. That's a pretty good balance of backwards compatibility versus new features. And you get to upgrade one file at a time.

Future versions of Perl promise they will support those version bundles for as long as technically possible. This is why a module that starts with use v5.16; still works fine when run by perl5.40.0.

Now that you're conviced that use VERSION is the way to go, you might want to start using it everywhere.

Picking which version to use is balancing act, with different choices for different situations. A CPAN author will prefer to support a wider range of Perl versions and limit themselves to somewhat older versions, while someone writing code for their company's e-commerce web site will want to be on the bleeding edge, and take advantage of the latest improvements to the language.