Based on David Cook's correct point on Bug 20104, revert the version, but not the refactor, to 5.10 for perl.
Created attachment 74322 [details] [review] Bug 20594: Revert Perl version to 5.10 While hunting perldoc initially, caller could not be found prior to 5.20.x Perl. However, David Cook pointed out it has existed since 5.8.8 of Perl. So, this reverts the version back to 5.10, while keeping the refactor that requires only changing installer/install.pl in the future for Perl version changes.
I am going to revert bug 20104 to make the release notes clear.
Okay, then can I squash them both and call the refactor an enhancement? Clearly, touching the version number is up for discussion on koha-devel.