Here's a pure perl way to do it (Check it. I may have misunderstood you).

There several changes that would speed this up, but if you want the absolute fastest way, you'd be better looking at Inline::C for this.

sub toMidi {
    my $out = '';
    for my $batch ( unpack '(a7)*', $_[ 0 ] ) {
        $batch .=  chr(0) x ( 7 - length( $batch ));
        my @bytes = unpack 'C7', $batch;
        my $h = 0;
        $h = ( $h >> 1 ) | ( 0x80 & $bytes[ $_ ] ) for 0 .. 6;
        $bytes[ $_ ] &= 0x7f for 0 .. 6;
        $out .= pack 'C8', $h >> 1, @bytes;
    }
    return $out . chr( 0xf7 );
}
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