betterworld sad:

Apparently, this special variable is only a string in perl 5.10, even though the documentation seems to suggest that it be a floating point number. If you use $]+0 in your example, it'll work (which is kind of strange because ordinary hash keys are converted to strings anyway, but in this case I think we are dealing with a tied hash).

Thanx, that's solved it. I will read about tied hashes and see if this is a problem.