my @start = ([],[],[]);
my $sequence = 'ATGATGAATGAATAGTAGATAG';
for my $i (0 .. length($sequence)-3) {
   my $triplet = substr($sequence, $i, 3);
   if ($triplet eq 'ATG') {
      push @{$start[$i%3]}, $i;
   }
   elsif ($triplet eq 'TAG' || $triplet eq 'TAA' || $triplet eq 'TGA') {
      for my $j (@{$start[$i%3]}) {
         print $j, '..', $i+2, "\n";
      }
      @{$start[$i%3]} = ();
   }
}

##</code><code>##

0..14
3..14
7..21

##</code><code>##

ATGATGAATGAATAGTAGATAG
STA123123123END
   STA123123END
       STA123123123END