If I had the chance, I would rewrite some_sub to work on lists, which would allow me to say

But I will grant you that that is a somewhat facetious answer, as I understand your question to mean an arbitrarily complicated map code block and not something as simple.

But then were the map code block arbitrarily complicated, I would no longer be using map, but instead I would be using for, which would make the code look like:

my @result;
for my $thing(qw(a b c d)) {
  # complex code block that munges $thing followed by
  push @result, $thing;
}
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On the Principle of Least Surprise, by and large I tend to put unsurprising code in maps and greps. That rules out function calls and other invisible things that might play around with $_. for blocks do not suffer from these problems because you can name your own topicalizer (or whatever it's called) with the for my $topic construct. If you stick to this, you can't be bitten by the types of problems you describe.

<update> if you have code running around secretly overloading the + operator, then clobbering $_ is likely the least of your problems. Just because you can write code that dicks around with $_, doesn't mean you should. We'll just have to agree to disagree. Your point is valid and I find no fault with it, I just see things differently.</update>