It's not the (purely ęsthetic) switch from \n to \x0A that solves the problem, it's the redo.

I'm aware of that, but the casual reader might not be, and misinterpret your post to mean that the fix for the U+0Axx-U+yy00 issue is the change from "\n\0" to "\x0A\x00" — which is why I thought it's worth pointing it out...

...about map $_ & 0xff, being worthless (harmful?)

...harmful only if there actually are characters outside of the latin1 range (which didn't seem to be the case with the input presented by the OP). And even then the "harm" is just an occasional incorrect arbitrary character instead of a "?" (so why worry, with a crude hack?).

As to being worthless, it at least makes explicit what the intention is, and avoids the warning "Character in 'C' format wrapped in pack" with newer versions of Perl when the value is greater than 0xff (and when warnings are enabled). You're right however in that with 5.6.1 it's worthless (strictly speaking), as 5.6.1 does not issue a warning in this case, but rather silently truncates the value.