Test your code at least on the minimum and maximum version of Perl you want to support (and if you want to be thorough, the ones in between too).

You can easily have different Perl versions installed on your system. See for example perlbrew.

If by "downgrade" you mean by some kind of option to make a newer Perl behave like an older one, then no, at least not completely: While you can disable features, that won't disable all of the things that the newer Perl supports. Only actually using the older Perl version will give you reliable test results.