favor `do {}' over `eval {}' for localized slurp

favor `do {}' over `eval {}' for localized slurp

I did not know to use the return value of `do' back in the day.
There's probably no practical difference in these cases, but
`eval' is overkill for these uses and may hide actual errors.

We can get rid of a few redundant `scalar' ops and pass scalar
refs to Email::MIME->new to avoid copies in a few more places,
too.