The Camel Book, edition 3, page 735: "A SIGNAL of 0 tests whether a process is still alive and that you still have permission to signal it. No signal is sent. This way you can check whether the process is still alive and hasn't changed it's UID."

If you want to lose the assumption about UID, but add the assumption that you're working on a system with a proc file system, you could instead do the following...

$path = "/proc/$pid";
print "process $pid ", -e $path ? "is " : "is not ", "running\n"
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This may or may not be faster, depending on how much "system-callage" the various tests engender, and the relevance of that depends on whether this is a monitoring daemon firing off all fast and furious, or if it's a command line executed script that just does the test once.