You don't mention what kind of variable you are tying, though you do mention scalar later in your post, so this extract from the Perl 5.8 delta pod may or may not be relavent.

Self-tying Problems

Self-tying of arrays and hashes is broken in rather deep and hard-to-fix ways. As a stop-gap measure to avoid people from getting frustrated at the mysterious results (core dumps, most often), it is forbidden for now (you will get a fatal error even from an attempt).

A change to self-tying of globs has caused them to be recursively referenced (see: Two-Phased Garbage Collection in the perlobj manpage). You will now need an explicit untie to destroy a self-tied glob. This behaviour may be fixed at a later date.

Self-tying of scalars and IO thingies works.