Instead of doing a split, doing line by line processing makes more sense. Just create a single regex to detect what looks like a continued line to you.

while (<DATA>) {
    chomp;

    # Continued Line
    if (/^\s+(.*)(?<!:)$/) {
        print " $1";

    # New Line
    } else {
        print "\n" if $. > 1;
        print $_;
    }
}
print "\n";

__DATA__
Into the little hill
2-osainen lyyrinen tarina sopraanolle, altolle seka viidelletoista
soittajalle
    1. OSA (01-05) /20:28:
01  1. The crowd (Kill them they bite, kill them they steal -) /0:50.
02  2. The minister and the crowd (The minister greets the crowd -)
    /2:50.
03  3. The crowd (Kill them they bite, kill them they steal -) /1:42.
04  4. The minister and the stranger (Night comes but not sleep -)
    /8:33.
05  5. Interlude - Mother and child (Why must the rats die, Mummy? -)
    /6:33.
    2. OSA (06-08) /16:34:
06  6. Inside the minister's head (Under a clear sky, the minister
    steps from the limousine -) /3:43.
07  7. The minister and the stranger (His head lies on his desk,
    between the family photograph -) /5:52.
08  8. Interlude - Mother(s) and child(ren) (Each cradle rocks empty
    -) /6:59.
    3. OSA BLAH BLAH BLAH BLAH BLAH BLAH BLAH BLAH BLAH BLAH BLAH
    BLAH BLAH BLAH (09-10) /14:14:
09  9. Another very long stupid song title to be used as yet another
    dumb example /6:66.
10  10. Last fictive song title /6:66.
[download]

Into the little hill
2-osainen lyyrinen tarina sopraanolle, altolle seka viidelletoista
soittajalle
    1. OSA (01-05) /20:28:
01  1. The crowd (Kill them they bite, kill them they steal -) /0:50.
02  2. The minister and the crowd (The minister greets the crowd -) /2
+:50.
03  3. The crowd (Kill them they bite, kill them they steal -) /1:42.
04  4. The minister and the stranger (Night comes but not sleep -) /8:
+33.
05  5. Interlude - Mother and child (Why must the rats die, Mummy? -) 
+/6:33.
    2. OSA (06-08) /16:34:
06  6. Inside the minister's head (Under a clear sky, the minister ste
+ps from the limousine -) /3:43.
07  7. The minister and the stranger (His head lies on his desk, betwe
+en the family photograph -) /5:52.
08  8. Interlude - Mother(s) and child(ren) (Each cradle rocks empty -
+) /6:59. 3. OSA BLAH BLAH BLAH BLAH BLAH BLAH BLAH BLAH BLAH BLAH BLA
+H
    BLAH BLAH BLAH (09-10) /14:14:
09  9. Another very long stupid song title to be used as yet another d
+umb exampl
e /6:66.
10  10. Last fictive song title /6:66.
[download]

However, I do see one problem with your stated logic after '08' in your sample date, as I suspect you want the following to appear on its own line:

    3. OSA BLAH BLAH BLAH BLAH BLAH BLAH BLAH BLAH BLAH BLAH BLAH BLAH
+ BLAH BLAH (09-10) /14:14:
[download]

while (<DATA>) {
    chomp;
    
    # Continued Line
    if (/^(?>\s+)(?!\d+\.)(.*)$/) {
        print " $1";

    # New Line
    } else {
        print "\n" if $. > 1;
        print $_;
    }
}

print "\n";
[download]

Into the little hill
2-osainen lyyrinen tarina sopraanolle, altolle seka viidelletoista
soittajalle
    1. OSA (01-05) /20:28:
01  1. The crowd (Kill them they bite, kill them they steal -) /0:50.
02  2. The minister and the crowd (The minister greets the crowd -) /2
+:50.
03  3. The crowd (Kill them they bite, kill them they steal -) /1:42.
04  4. The minister and the stranger (Night comes but not sleep -) /8:
+33.
05  5. Interlude - Mother and child (Why must the rats die, Mummy? -) 
+/6:33.
    2. OSA (06-08) /16:34:
06  6. Inside the minister's head (Under a clear sky, the minister ste
+ps from the limousine -) /3:43.
07  7. The minister and the stranger (His head lies on his desk, betwe
+en the family photograph -) /5:52.
08  8. Interlude - Mother(s) and child(ren) (Each cradle rocks empty -
+) /6:59.
    3. OSA BLAH BLAH BLAH BLAH BLAH BLAH BLAH BLAH BLAH BLAH BLAH BLAH
+ BLAH BLAH (09-10) /14:14:
09  9. Another very long stupid song title to be used as yet another d
+umb example /6:66.
10  10. Last fictive song title /6:66.
[download]