I might get nervous, but not extremely nervous. Subs (and methods) shouldn't clobber $_ unless they're documented to — and with a purpose. For every line in your block it should be clear to what exactly it does to $_.

In such a case, multiline blocks using $_ are fine.

p.s. Your anxiousity doesn't warrant to be restricted to just for loops. You seem to be wary of anything messing with $_, anywhere. Probably you never use $_ over several lines.