You can turn on strict with use 5.12.0; or any explicit later version, but that's not a great way of just turning on strict because you may get unexpected side effects or limit functionality when using newer version of Perl.

I recommend to add a use $VERSION; anyway, but not 5.12.0 unless you've actually tested your code under that version. It doesn't limit functionality: If you need functionality from a newer version of Perl, you're well advised to adjust to that version in the use statement.

Unexpected side effects can occur if you add a version declaration to legacy code which hadn't one, but if you start writing your code with a version declaration, you can look it up what side effects are to expected from bumping that version.