I had no idea that my attempt at obfuscation would actually lead to something, as I was just using underscore as a variable name to amuse myself. From open:

If EXPR is omitted, the scalar variable of the same name as the FILEHANDLE contains the filename. (Note that lexical variables--those declared with "my"--will not work for this purpose; so if you're using "my", specify EXPR in your call to open.) See the perlopentut manpage for a kinder, gentler explanation of opening files.