For #311, @subbuss wrote the following comment, which describes the work needed for this issue:

The reason -X+C doesn't have this bug is because I did not clean up the JIT to remove the $! hack as I did with -X-CIR and -X-C (See commit log note in 7c3f642). Curious why this example doesn't fail in -X-CIR mode since IR doesn't have the $! hack either. I do know that in IR mode, $! is saved and restored properly in IR-generated code, so maybe the clobbering in the runtime does not matter ... but just a guess.

The bigger issue that this bug exposes is that "$!" is a proxy for the errorInfo value in ThreadContext and so, $! can be modified indirectly by directly calling setErrorInfo. Ideally, in later commits, we should do the following:

• clean up the JIT to get rid of the $! hack as done for the AST interp in 20632af
• audit all uses of setErrorInfo to see if they clobber $! accidentally anywhere else.
• unify the globalVariables.set("$!", foo) and setErrorInfo(foo) paths so that we use one or the other uniformly, if feasible.