from my version of the open documention (perl 5.6.1) but not the online monks version of the open doc -

In the 2-arguments (and 1-argument) form of the call the mode and filename should be concatenated (in this order), possibly separated by spaces.

so you can separate the mode from the filename by a space (and the code in doio.c backs that up) but you're right, it's not idiomatic.

-derby