f there is an example of undef not being undef other than the previously mentioned open hack, it's well hidden. If you believe differently, there's gotta be some obvious example I'm missing.
undef $foo;
utime $foo, $foo, "file";   # Sets atime, mtime to 00:00:00 Jan 1, 197
+0
utime undef, undef, "file"; # Sets atime, mtime to time().

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In reply to Re^14: Use of uninitialized value in substr by JavaFan
in thread Use of uninitialized value in substr by BrowserUk

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