How to delete a file named "filen/ame" (with slash) on an ext4 filesystem in debugfs?

If you want a fix and are not just trying out debugfs, you can have fsck do the work for you. Mark the filesystem as dirty and run fsck -y to get the filename changed:

$ debugfs -w -R "dirty" /tmp/ext4fs
$ fsck -y /tmp/ext4fs
 ...
/tmp/ext4fs was not cleanly unmounted, check forced.
Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Entry 'filen/ame' in / (2) has illegal characters in its name.
Fix? yes
 ...
$ debugfs -w -R "ls" /tmp/ext4fs
2  (12) .    2  (12) ..    11  (20) lost+found    12  (980) filen.ame   

Congradulations on fsck working; if for some reason it didn't work the answer is ls -i1 followed by umount and then clri.

Ref: http://docstore.mik.ua/orelly/unix/upt/ch23_13.htm

I have actually tested this method.