use warnings;
use strict;
my $date;
my $lastdate = '';
my $line = 0;

while(<DATA>){
   $line++;
   $date = $_;
   if (($lastdate ne $date) && ($lastdate ne '')){
      print "$line: $lastdate";
   }
   $lastdate = $date;
}
print "$line: $lastdate";
<>;
__DATA__
10
10
10
11
11
11
12
12
12

##</code><code>##

4: 10
7: 11
9: 12