The problem was a typo when setting up the DB connection. Fixed that, started getting an error on line 143 of Apache::Session::DBIStore.

So I added some code before the offending line:

### SDW DEBUG
    $self->{insert_sth}->trace(9, "/tmp/sessioncheck");
    open(SDW_DEBUG_OUT, ">/tmp/serial");
    print SDW_DEBUG_OUT "FOO".$serialized."BAR\n";
    close(SDW_DEBUG_OUT);
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And I get:

FOO
bc6f80d5a358ae0f_session_idBAR
[download]

Note the linefeed between FOO and the start of the sessionid. The serialized data seems to start with a linefeed! Needless to say, this foobars the sql statement.

This can't be right, can it?