events_r_to_score_r

Hello, happy new year !

I'm trying to convert several tracks from a MIDI file into an alone one. (The 
ultimate goal is to convert MIDI file to MDA files for my old yamaha DRC-20)

My question : why "events_r_to_score_r" doesn't work in the code below:
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#!perl -w
use strict;
use MIDI;
my $event_r = ['note_on', 0, 0, 60, 64];
print "$_, " for @$event_r; print "\n"; # ok: note_on, 0, 0, 60, 64,
my $score_r = MIDI::Score::events_r_to_score_r( $event_r );
print ref $score_r, "\n"; # ARRAY
print scalar @$score_r, "\n"; # 0
----------------

Subsidiary question: is there another way to convert a multi-track MIDI file 
to a one-track one?

Thanks for help,

Jean-Pierre