A for (@ary) {} is like for local $_ (@ary) {}. So when using for(?:each)? loops it is safe to nest as much as you like......but

for loops are almost the only thing which automatically localises $_. While loops do not. New scopes do not. New packages/files do not. maps and greps do. foreach does. Three argument for does not. And theres probably more things I should be listing.

for (@files) {
  open FILE, $_;
  while (<FILE>) {
    push @res, grep {ord($_)<128} split //, $_;
  }
  close FILE;
}
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Be careful, know what's happening and don't overuse $_ where using sensible variable names would be more sensible.