The 'use if' line seems to work only if written as follows:

use if ($^O eq 'linux'), 'MIDI::ALSA' => ('SND_SEQ_EVENT_PORT_UNSUBSCR
+IBED',
                                          'SND_SEQ_EVENT_SYSEX');
[download]

Any other way to write the 'use if' CONDITION doesn't seem to work.
Weird...