It appears, when Perl prints to binary STDOUT, it tries to encode some strings as Latin-1

No. When you don't specify an encoding, print expects bytes, and prints those bytes provided without encoding.

$ perl -e'print pack "C*", 0..255;' | od -t x1
0000000 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
0000020 10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f
0000040 20 21 22 23 24 25 26 27 28 29 2a 2b 2c 2d 2e 2f
0000060 30 31 32 33 34 35 36 37 38 39 3a 3b 3c 3d 3e 3f
0000100 40 41 42 43 44 45 46 47 48 49 4a 4b 4c 4d 4e 4f
0000120 50 51 52 53 54 55 56 57 58 59 5a 5b 5c 5d 5e 5f
0000140 60 61 62 63 64 65 66 67 68 69 6a 6b 6c 6d 6e 6f
0000160 70 71 72 73 74 75 76 77 78 79 7a 7b 7c 7d 7e 7f
0000200 80 81 82 83 84 85 86 87 88 89 8a 8b 8c 8d 8e 8f
0000220 90 91 92 93 94 95 96 97 98 99 9a 9b 9c 9d 9e 9f
0000240 a0 a1 a2 a3 a4 a5 a6 a7 a8 a9 aa ab ac ad ae af
0000260 b0 b1 b2 b3 b4 b5 b6 b7 b8 b9 ba bb bc bd be bf
0000300 c0 c1 c2 c3 c4 c5 c6 c7 c8 c9 ca cb cc cd ce cf
0000320 d0 d1 d2 d3 d4 d5 d6 d7 d8 d9 da db dc dd de df
0000340 e0 e1 e2 e3 e4 e5 e6 e7 e8 e9 ea eb ec ed ee ef
0000360 f0 f1 f2 f3 f4 f5 f6 f7 f8 f9 fa fb fc fd fe ff
0000400
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That means,

• If you provide Unicode code points, you will get Unicode code points.
• If you provide latin-1, you will get latin-1.
• If you provide latin-2, you will get latin-2.
• If you provide gzipped data, you will get gzipped data.
• etc

In your example, 70:114:97:110:231:97:105:115 are the Unicode code points that formed "Français". It's just that the latin-1 encoding of the first 256 code points is itself.

$ perl -MEncode=encode -E'
   $_ = pack "C*", 0..255;
   say $_ eq encode("iso-latin-1", $_) ? "same" : "diff";
'
same
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Exception: If any of the characters it he string are not bytes (larger than 255), print will assume you forgot to specify :utf8. it will warn ("wide character") and encode the characters accordingly.