The problem is that when I do a length on the return value. Of course I should have used "bytes", but as I said, the return value is a binary string, so returning a length in utf8 characters is strange.
And what's great with this bug, is that you only see it when the original string has multi-bytes characters or when it is long enough. :) 
use Encode qw/_utf8_on/;
my $a="bj\xc3\xb6rk";
_utf8_on($a);
my $binarystring=pack("V/a*", $a);
warn length $binarystring;
warn bytes::length $binarystring;

my $b="b"x1000;
_utf8_on($b);
my $binarystring2=pack("V/a*", $b);
warn length $binarystring2;
warn bytes::length $binarystring2;

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In reply to Re^3: 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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