Because you're modifying the array you're iterating over which is a no-no. See perlsyn for the details, but the cliffs notes is use while if you're modifying the array in place just as you've discovered.

Update: OK, it's not really extensive details; here's the warning in its entirety:

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.