Optimizations in _parse_fh

[dolmen]

Multiple small optimizations.

[karenetheridge]

Thanks for this PR, and I will review it, but Module::Metadata is undergoing a near-complete rewrite right now, so performance optimizations are a bit premature and will need to be redone afterwards anyway :)

Thanks for the poke though, and I'll endeavour shortly to return to my work here that was being done at QAH.

[miyagawa]

@dolmen looks like this PR is a bit controversial: guess you can make a separate PR for just utf-8'ing source code? 64d7abb

[dolmen]

Summary of remaining commits to rework:

• 145dec760
• 762356c02