my @sorted =
    sort {
       my $count_a = $a =~ tr/GC//;
       my $count_b = $b =~ tr/GC//;
       $count_a <=> $count_b
    }
    @sequences;

##</code><code>##

my @sorted =
    map substr($_, 4),
    sort
    map pack('N', tr/GC//) . $_,
    @sequences;