use strict; use warnings;
my @bas = ("A","G","C","T","U");
my @all = ();

for my $a1(@bas){
 for my $a2(@bas){
   for my $a3(@bas){ push @all, "$a1$a2$a3\n"}}

my @codon = grep !/(U.*T|T.*U)/, @all; 
      for my $valid(@codon){ print $valid }
}