Ok, I'll try to be clear this time :)
What I wanted is write the string encoded in utf8, and the length, in bytes, of the binary string resulting from pack. So I was using : 
my $p=pack "V/a*", $s;
my $l=length $p;

[download]
When I should have been using : 
use Encode qw/encode/;
my $p=pack "V/a*", encode('utf8',$s);
my $l=bytes::length $p;
# using bytes::length just to be sure, $p shouldn't have its utf8 flag
+ on, but in case it does...

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Thinking about it a little more, I think what is disturbing me is that the 'a' in the pack format can be a multi-bytes character. And more generally, the idea that utf8 strings are strings of multi-bytes characters, rather than strings of bytes in utf8 encoding.
perl 5.10's pack behavior does seem to make more sense now.

In reply to Re^7: Understanding pack and unpack changes for binary data between 5.8 and 5.10 by squentin
in thread Understanding pack and unpack changes for binary data between 5.8 and 5.10 by dkg

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