my @sorted = (sort {$count{$a} cmp $count{$b}} keys %count);
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$count{$a} and $count{$b} contain numbers so that should be:

my @sorted = sort {$count{$a} <=> $count{$b}} keys %count;
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And you could accomplish the same effect with just one loop:

$ 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
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