As BrowserUK suggested, I put a die statement on the open, and found that /dev/sequencer didn't exist, although it's there, and the permissions are OK.

I started to think that MIDI::Realtime was written for OSS and it isn't quite compatible with ALSA's OSS compatibility mode.

I did is some WildAs_ experimenting and changed /dev/sequencer to /dev/amidi and it "seemed to run" without error, and I could hear an elevated level of static in my FM radio, so I think it's putting out, except there is no output to the soundcard.

It would seem that it should be simple to write to the sequencer? The utilities which come with alsa work well, but they seem to rely on port numbers, which I can't seem to fit anywhere in MIDI::Realtime.