It's not "unpredictable" in the sense of being random but it is undefined in the sense that it is not specified in the documentation and could potentially change in a future release. Quoting perlsyn:

If any part of LIST is an array, foreach will get very confused if you add or remove elements within the loop body, for example with splice. So don't do that.

I should add for the record that I once asked the same question myself.