#!/usr/bin/perl --
use warnings;
use strict;
use re 'debug';

my $dna = 'atctcggataatgggataaaaatataggctataaatggcgccccggctaattttt';

if ($dna =~ /atg([acgt]+)(?!(taa|tag|tga))/xms)
{
    print $1;
}
__END__

$ perl fudge
Compiling REx "atg([acgt]+)(?!(taa|tag|tga))"
Final program:
   1: EXACT <atg> (3)
   3: OPEN1 (5)
   5:   PLUS (17)
   6:     ANYOF[acgt][] (0)
  17: CLOSE1 (19)
  19: UNLESSM[0] (36)
  21:   OPEN2 (23)
  23:     EXACT <t> (25)
  25:     TRIE-EXACT[ag] (32)
          <aa>
          <ag>
          <ga>
  32:   CLOSE2 (34)
  34:   SUCCEED (0)
  35: TAIL (36)
  36: END (0)
anchored "atg" at 0 (checking anchored) minlen 4
Guessing start of match in sv for REx "atg([acgt]+)(?!(taa|tag|tga))" against "atctcggataatgggataaaaatataggctataaatggcgc
cccggctaattttt"
Found anchored substr "atg" at offset 10...
Starting position does not contradict /^/m...
Guessed: match at offset 10
Matching REx "atg([acgt]+)(?!(taa|tag|tga))" against "atgggataaaaatataggctataaatggcgccccggctaattttt"
  10 <ggata> <atgggataaa>    |  1:EXACT <atg>(3)
  13 <taatg> <ggataaaaat>    |  3:OPEN1(5)
  13 <taatg> <ggataaaaat>    |  5:PLUS(17)
                                  ANYOF[acgt][] can match 42 times out of 2147483647...
  55 <cggctaattttt> <>       | 17:  CLOSE1(19)
  55 <cggctaattttt> <>       | 19:  UNLESSM[0](36)
  55 <cggctaattttt> <>       | 21:    OPEN2(23)
  55 <cggctaattttt> <>       | 23:    EXACT <t>(25)
                                      failed...
  55 <cggctaattttt> <>       | 36:  END(0)
Match successful!
ggataaaaatataggctataaatggcgccccggctaatttttFreeing REx: "atg([acgt]+)(?!(taa|tag|tga))"