$! only has meaning if the operation failed. After a success its value could be anything.

perldoc says: If used numerically, yields the current value of the C errno variable, or in other words, if a system or library call fails, it sets this variable. This means that the value of $! is meaningful only immediately after a failure:

In reply to Re^2: Socket descriptor passed across fork -- hanging by SuicideJunkie
in thread Socket descriptor passed across fork -- hanging by dd-b

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