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.

In reply to Re^5: self-feeding infinite loop by Errto
in thread self-feeding infinite loop by spx2

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