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        #How does the print operator know what file handle to print to
+? 
        #Was this selected somewhere by magic?
        #Where is this documented / explained?

            From the shell, saying

...
            to know when the filename has changed.  It does, however, 
+use
            ARGVOUT for the selected filehandle.  Note that STDOUT is 
+restored
            as the default output filehandle after the loop.