/^(.*?)(?:, \1)*\z/;
my $count = (length($_)+2)/(length($1)+2);

##</code><code>##

for (
   '1, 2, 5, 1, 2, 5, 1, 2, 5',
   '3, 6, 3, 6, 3, 6, 3, 6',
   '4, 1, 28, 0, 4, 1, 28, 0',
   '3, 5, 17, 3, 17, 5',
   '4, 1, 28, 0, 4, 1, 28',
) {
   for (my $seq = $_) {
      /^(.*?)(?:, \1)*\z/;
      my $count = (length($_)+2)/(length($1)+2);
      print("$_: $count\n");
   }
}