Ok, I found out a solution.

The patch mentioned on the parent post has no effect for this particular case. I rolled back to DBD-mysql-3.0002. Actually, the fix consists of using the utf8 pragma, and then performing an upgrade in the string, like this:

use utf8;

...

my $string = "voilá monsieur";
utf8::upgrade($string);

# $string is now ready to be INSERTed by DBI,
# but you still need to do this

$dbhandle->do("SET CHARACTER SET utf8");

# now you can perform the INSERT
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