WHATS THIS

This tests that results are independent of the order in which various modules are 'use'd by the main program. It system()s a script which can vary order of inclusion based upon cmdline options

X.pm, Y.pm are essentially identical, and they must be, due to line-number checks done here. Z.pm also shares common line numbers, allowing same testing for output.

Z.pm uses a different alias, thus testing that multiple aliases are allowed (or at least not prohibited).

W.pm passes debug flags on its use-line, which affects debugging of all modules loaded afterwards (this is not specifically tested, but is evident in the stderr). This is not a bug, while I could probably localize the debug output for only that usage, debugging was never intended to have such selectivity.