...It didn't do that in perl 5.8

Another difference to be aware of is this:

my $s = "\x{1234}\x{5678}";   # string with utf8 flag on

print unpack("H*", $s), "\n";
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With 5.8 this prints a hexdump of the internal (UTF-8) representation of the string — e.g. useful when debugging encoding issues

e188b4e599b8
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while with 5.10, you'd get

3478
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i.e. the low-byte values of the codepoints, with the high-byte part being truncated. With warnings enabled, you also get "Character in 'H' format wrapped in unpack at...".

With use bytes, or when explicitly turning off the utf8 flag (update: as shown below), you get the old behaviour.  And specifically for debugging encoding issues, Devel::Peek is the recommended alternative since 5.10, because of this difference.