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

            From the shell, saying

                $ perl -p -i.orig -e "s/foo/bar/; ... "

            is the same as using the program:

                #!/usr/bin/perl -pi.orig
                s/foo/bar/;

            which is equivalent to

                #!/usr/bin/perl
                $extension = '.orig';
                LINE: while (<>) {
                    if ($ARGV ne $oldargv) {
                        if ($extension !~ /\*/) {
                            $backup = $ARGV . $extension;
                        }
                        else {
                            ($backup = $extension) =~ s/\*/$ARGV/g;
                        }
                        rename($ARGV, $backup);
                        open(ARGVOUT, ">$ARGV");
                        select(ARGVOUT);
                        $oldargv = $ARGV;
                    }
                    s/foo/bar/;
                }
                continue {
                    print;  # this prints to original filename
                }
                select(STDOUT);

            except that the -i form doesn't need to compare $ARGV to $
+oldargv
            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.
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