Other comments have covered various reasons why some code does eval $VERSION and other version number complications, but the local::lib test has some specific concerns. The test needs to be able to run with no non-core modules on perl versions going back to 5.6. This means tools like version.pm are not available to make the version comparisons easier. Additionally, the test needs to check the version of ExtUtils::MakeMaker, and several releases of perl shipped with versions of it that included underscores in the version.
So the intent is to convert a value like "6.57_05" into the number 6.5705 so that it can be compared numerically. This could be done in other ways to avoid using stringy eval, but given the use case there isn't really any danger from using an eval.