(again, everything in this reply is untested. Based purely on the problem description)

Ah, that one is another result of the current perl handlers. The order of execution is now:

 - signal happens, triggers perl internals
 - flag for this signal is set by internals
 - systemcall is interrupted, $! set to EINTR
 - perl dispatcher sees flag, starts your signal code
    However, this code happens to change $!
 - The assign of undef to $conn happens
 - You check $conn, see its undef and then go look at $!,
   but you get the $! left by your signal handler, not the
   older one you really need.
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Solved easily enough by adding

local $!;
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at the start of your signal handler.

Personally I consider this a perl flaw/bug though. I think perl should localize $! when it calls the deferred user signal code.