Nifty!

   perl dna TGAATCTTTGACTCGA
[download]

Hope I transcribed that correctly. I just hacked the following out of your code. Ugly, but does the job

#!/usr/bin/perl
# Run as dnareader.pl TCCCTCATTGAATCCAAACC
# You'll get a ++ from me if you can make it say "perl" 
# instead.

for my $g1 ( 'A', 'C', 'T', 'G' ) {
for my $g2 ( 'A', 'C', 'T', 'G' ) {
for my $g3 ( 'A', 'C', 'T', 'G' ) {
for my $g4 ( 'A', 'C', 'T', 'G' ) {
  print(  "$g1$g2$g3$g4 - " );
  letter( "$g1$g2$g3$g4" );
  print "\n";
}}}}

sub letter {
$"='ATCG';
foreach (split '', shift) {
 $;++;$,=$,<<2|index $",$_; 
 next if 
 $;&3;$,=print chr $,;$,--;
}
}
[download]

From there it's a simple matter of working backwards...

--

g r i n d e r
TACCTGTTTGAGTGTAACAATCATTCGCTCGGTGTATCCATCTTTGACACAATCACTCGGTCTCTCCA