treewide: enable warnings in all exec-ed processes

treewide: enable warnings in all exec-ed processes

While forked processes inherit from the parent, exec-ed
processes need the `-w' flag passed to them.  To determine
whether or not we should pass them, we must check the `$^W'
global perlvar, first.

We'll also favor `perl -e' over `perl -E' in places where
we don't rely on the latest features, since `-E' incurs
slightly more startup time overhead from loading feature.pm
(while `perl -Mv5.12' does not).