eval {
   ( my %session, $ok );
   $ok = tie( %session, 'Apache::Session::MySQL', undef, ... );
   if ( $ok )
   {
      ... do something with %session ...
   }
   untie %session;
}
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The trick here is to read the fine print on the return code provided by tie:

The object returned by the constructor is in turn returned by the tie function, which can be useful if you want to access other methods in CLASSNAME.

In other words, I can't untie %session because $ok is still holding a reference to it. One fix is to assign $ok to a constant if and only if tie returns a non-false value, e.g.:

eval {
   ( my %session, $ok );
   tie( %session, 'Apache::Session::MySQL', undef, ... )
         and $ok = 1;
   if ( $ok )
   {
      ... do something with %session ...
   }
   untie %session;
}
[download]

This eliminates the duplicate reference to %session, so my session properly goes away. Thank you each for the alternate solutions suggested by bart and the other monks in CB:

• if ( tie( ... ) ) { $ok = 1 }
• $ok = tie( ... ) && 1;
• undef $ok; (before untie %session)
and one particularly elegant answer, from tye:
• $ok = !! tie( ... );