#!/usr/bin/perl
# Run as dnareader.pl TCCCTCATTGAATCCAAACC
# You'll get a ++ from me if you can make it say "perl" 
# instead.
$"='ATCG';
foreach (split '', shift) {
 $;++;$,=$,<<2|index $",$_; 
 next if 
 $;&3;$,=print chr $,;$,--;
}