I've been banging my head on the table with Apache::Session::MySQL today. Consider:

eval {
   ( my %session, $ok );
   $ok = tie( %session, 'Apache::Session::MySQL', undef, ... );
   if ( $ok )
   {
      ... do something with %session ...
   }
   untie %session;
}
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This doesn't work from mod_perl, because the untie throws an exception following warning causes my web page to fail:

untie attempted while 1 inner references still exist at foo.pl line 250.

Since I don't use any explicit references to %session, this caught me by surprise. I have since resolved it; can you see the bug?

Updates:

• clarified failure per note from ysth. Occasionally I use an english-ish idiom instead of a precise technical term.
• The !! idiom may not be elegant to some, but I find it elegant because it can be read as "dammit," and I was looking for a -dammit flag to work around this failure :)
• The use of $ok instead of the simple if statement occurred because of some additonal logic that I was doing after the tie, and before the untie. It had been omitted in this sample code.