The trick with this sort of thing is to defer output of the current record until the decision can be made about the next record:

use strict;
use warnings;

my $record = '';

while (defined (my $line = <DATA>)) {
    chomp $line;

    if ($line =~ /^\S|:$/ && length $record) {
        print "$record\n";
        $record = '';
    }

    $record .= ' ' if length $record;
    $record .= $line;
}

print "$record\n" if length $record;

__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]

Prints:

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,     b
+etween 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]