bisect-runner.pl: Reword some pod examples

I found myself better able to understand what's really happening given
this rewording.
Add tests and clarify pod for (?[ ])

A compliled '(?[ ])' embedded in a larger one is unaffected by what
regex modifiers are in effect at the time of the compilation of the
outer one; it retains, going forward, the modifiers it had when it was
first encountered.