The consensus on p5p seems to be that it's more important for old programs to run unmodified on modern versions of Perl than for modern features of Perl to be available by default. In theory, use 5.10; would enable all of these features for 5.10 programs and use 5.12; for 5.12 programs, but the feature pragma at the heart of that behavior does not enable strict and warnings, and the version number keeps changing as what's "modern" changes.