I mainly want to start using the '//' (defined-or) operator to
simplify code, and Perl 5.10.1 is roughly a decade old at this
point.
"given/when" would've be nice, but it's future is in doubt AFAIK.
I also started using the 'parent' module in WwwHighlight, and
'autodie' in UserContent.pm, both of which were only distributed
with Perl since 5.10.1; and testing with ancient
versions/distros is time-consuming.
Anyways, I think this a small-enough jump to not break any
existing installations, given we already depend on fairly
recent versions of git and Xapian.
Maybe we can use more newish Perl features in the future...