The use 5.010: works only with version 5.10 of perl and newer, and 5.10 was released in December 2007.

So whoever wants their code to run on older perls doesn't use it, but misses a few features (say, given/when, smart-matching, //= etc. The latter two don't require the 'use 5.010;', but require a 5.10 or newer nonetheless).

I also sometimes omit it because I'm simply not yet used to it, and while my finger type use strict; use warnings; mechanically without thinking, the same doesn't hold true for requiring newer perl versions.