There error is happening because the module MIDI::ALSA is not installed on your Windows machine.

use doesn't work the way you're thinking it does. It is not evaluated as the code is run, but before that. So your "logic" is correct, but your logic is wrong - if that makes sense ;-)

There have been many suggestions on how to do this with the if pragma for example, but I've found this to work for me:

use strict;
use warnings;

my $HAVE_MIDI_ALSA = 0;
eval "use MIDI::ALSA";
if($@) {
    print "MIDI::ALSA required\n";
    exit
} else {
    $HAVE_MIDI_ALSA = 1
}

if (($^O eq 'linux') and $HAVE_MIDI_ALSA) {
    MIDI::ALSA::client('test',1,1,1);
    MIDI::ALSA::connectfrom(0,'MidiSport 4x4:0');
    MIDI::ALSA::connectto(1,'MidiSport 4x4:0');
    MIDI::ALSA::start();
}
[download]