Action-at-a-distance was precisely the intention in this case. One might then trivially enhance a standard capture with a certain additional effect like dropping of privileges. Callbacks like that allow for a (more) generalized routine instead of a bunch of specialized modules.

Anyway, I was contemplating the numerous problems with piping/capturing I've witnessed on PM and elsewhere. Can you give an example where the list form open has caused mayhem, because of the one-element list?

As far as qx{}; is concerned, I do not really see any problem. The string inside qx is not perl code, it is shell syntax. One could perhaps make a point about always requesting a shell, even when perl thinks this is redundant, like qx{}F; maybe. The opposite, to force an op to not do what it's intended to do, makes no sense.

Edit. BTW: out of curiosity, do you sometimes use the <> operator in your code or do you always go for the safe diamond? I'd have simply plugged *that* hole, methinks...

Edit2. Clarification in regards to "shell syntax". The qx is for interfacing with system shell, the meaning is thus "system shell syntax, whatever that may be". Never mind about the qx though, I just found your safe_qx() oddly named, that's all.