Here's a swing, strict at 222:

sub f {
  my@r=qw(UA[AG]|UGA GC. - UG[UC] GA[UC] GA[AG] UU[UC] GG. CA[UC] AU[^
+G] - AA[AG] CU.|UU[AG]
  AUG AA[UC] - CC. CA[AG] CG.|AG[AG] UC.|AG[UC] AC. - GU. UGG - UA[UC]
+);
  ((my$t=pop)=~s|...|chr 64+(grep$&=~/$r[$_]/,0..25)[0]|eg);$t=~y/@/./
+;$t
}
[download]

update: I can't count, the one above is actually 232. And as no_slogan points out, it's sometimes helpful to read the spec. 238 chars:

sub f {
  my@r=qw(UA[AG]|UGA GC. - UG[UC] GA[UC] GA[AG] UU[UC] GG. CA[UC] AU[^
+G] - AA[AG] CU.|UU[AG] AUG AA[UC] - CC. CA[AG] CG.|AG[AG] UC.|AG[UC] 
+AC. - GU. UGG - UA[UC] ^);
  ((my$t=pop)=~s|..?.?|chr 64+(grep$&=~/$r[$_]/,0..26)[0]|eg);$t=~y/@Z
+/./d;$t
}
[download]

   MeowChow                                   
               s aamecha.s a..a\u$&owag.print