where did you assigned value to $pid ? :-)

oops! open returns it. I'll fix it.

You can also miss some data, for example @cmd can handle TERM signal and print partial result.

Change the die into a warn if you want to handle partial results.

Actually, you're the one who misses data. <> is a buffered read, so you'll lose whatever's in the buffer if a timeout occurs. Or maybe it's recoverable by doing another read after the timeout occurs?

@cmd also can block/not handle TERM signal (or in some case, cannot be KILLed).

The killing mechanism can be expanded to add a kill KILL => $pid; if the process doesn't end after a certain time. That's easy, and it's outside of what the OP was asking about.