This is what I've done in the past.

use Time::HiRes 'sleep';
use Win32::MediaPlayer;
sub sound_play {
    my $winmm = Win32::MediaPlayer->new();
    $winmm->load($alarm_sound);
    $winmm->play();
    my $pos = 0;
    (-$pos + ($pos = $winmm->pos)) or return while sleep 0.1;
}
[download]

The relative order of load and play and pos are important here, but I've forgotten why. I think it worked fine in a forked process.