my @set = qw (AAAAAT ATCGAT TTTTTG GCCCCC GTGGGG);
my $lim = 0.75;

my @newset = grep {

    my $a = $_ =~ tr/A//;
    my $t = $_ =~ tr/T//;
    my $c = $_ =~ tr/C//;
    my $g = $_ =~ tr/G//;

    $a / $len < $lim
       and $c / $len < $lim
       and $g / $len < $lim
       and $t / $len < $lim;

} @set;