Ok, so it looks like the rules are:

• ATG can be be a start codon, or it can code for Methionine.
• * TAG, TAA, and TGA are always end codons.
• * A paired start and end have to be separated by a multiple of three bases.

Here's a one-pass solution:

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]} = ();
   }
}
[download]

Output:

0..14
3..14
7..21
[download]

Explanation of the output:

ATGATGAATGAATAGTAGATAG
STA123123123END
   STA123123END
       STA123123123END
[download]