I guess your SQL raises an error that you never ask for. I suggest you initialize your database connection via

my $dbh = DBI->connect(..., {RaiseError => 1});
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that way, your program will die whenever it encounters an SQL error. Alternatively, you could check for errors in your SQL by modifying your code to:

$dbh->do(qq|set names 'utf8'|)
    or die "Could't set names to utf8: " . $dbh->errstr;
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but it can be hard to track down every unguarded usage of a DBI method. (As an aside, maybe a DBI::fatal would be useful here). Your unguarded usage of $dbh->do leaves the errstr value in it and then triggers a bug in CGI::Session, which seems to always assume that it can treat any serializer like a DBI / dbh / sth object, but obviously the ::storable subclass doesn't behave like a DBI class.

You might want to raise a bug report with the CGI::Session maintainer(s), as I find many calls to ->errstr scattered throughout the code, so I guess that part of the code was never tested against the ::storable serializer.