$ perldoc -f fileno

    fileno FILEHANDLE
            Returns the file descriptor for a filehandle, or undefined
+ if the
            filehandle is not open. If there is no real file descripto
+r at the
            OS level, as can happen with filehandles connected to memo
+ry
            objects via "open" with a reference for the third argument
+, -1 is
            returned.
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