I would think you could use some sort of grep. Here's kindof what I was thinking

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;
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Update:
Wow... I just got beat to the punch big time! Three people posted while I was writing my response!

    -Bryan