Take a look at the debugger 'w' command -- set global watch point. (Sorry, I don't know which release of Perl brought it in; it's in the 5.8.x versions and I don't remenber it using it with 5.6.x.)

I don't know how it will work against a reference, you may have to watch the base variable. ('w %hash'rather than 'w \%hash').