You're right, I also don't know. Most of the time, I do explicitly close my file handles, even if it is not necessary (at least, it is self-documenting), but I don't ever check whether closing the file was successful. Quite often, I close a bunch of files in one line such as:

close $_ for qw/$IN1 $IN2 $OUT1 $OUT2/;
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(Except that this is just an example, I am usually trying to give more useful names to my file handles.)