$ perl -le'
my @strings = qw( QS DD DFD SD QS QS QS DD DFS );
my ( %count, @max );          
for ( @strings ) {
    $count{ $_ }++;
    $max[ 0 ] < $count{ $_ } and @max = ( $count{ $_ }, $_ );
    }
print $max[ 1 ];
'
QS