my @set = qw (AAAAAT ATCGAT TTTTTG GCCCCC GTGGGG);
my $lim = 0.75;
my $len = length $set[0];

print "BEFORE:",scalar(@set),"\n";

map  { ( ((tr/A//)/$len < $lim) &&
          ((tr/T//)/$len < $lim) &&
          ((tr/C//)/$len < $lim) &&
          ((tr/G//)/$len < $lim)    ) or $_ = '' } @set;

print "AFTER:",scalar(@set),"\n";