From perlvar (5.8.0):
ARGVOUT

The special filehandle that points to the currently open output file when doing edit-in-place processing with -i. Useful when you have to do a lot of inserting and don't want to keep modifying $_. See perlrun for the -i switch.

Note: While it is mentioned in the Perl documentation prior to version 5.8.0, I have yet to see it explained in such a way.