Correct. unicode_strings affects things that care whether characters in the U+80 to U+FF range are letters or not, or whitespace or not. This includes uc, split and the regex engine. It doesn't affect I/O.

$ t() {
   perl -e'
      use open ":std", ":encoding(UTF-8)";
      use if $ARGV[0], feature => "unicode_strings";
      CORE::say uc "\xE9";
   ' -- "$@"
}

$ t 0
é

$ t 1
É
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$ t() {
   perl -e'
      use open ":std", ":encoding(UTF-8)";
      use if $ARGV[0], feature => "unicode_strings";
      CORE::say join "|", split " ", "a\x20b\xA0c";
   ' -- "$@"
}

$ t 0
a|b c

$ t 1
a|b|c
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