The handling of arguments masked with ' (ASCII char 0x27) has changed in 5.1 on Windows. QCoreApplication will report the original PATH, while Qt 5.1 stripped the ':
run with 5.1:
Since this might possibly affect a lot of applications, it should at least be documented. The change of behavior is most likely caused by https://codereview.qt-project.org/#change,64127 .