>perl -wMstrict -le
"my $dna = 'atctcggataatgggataaaaatataggctataaatggcgccccggctaattttt';
 ;;
 my @sub_seqs;
 push @sub_seqs, [ ${^MATCH}, $-[0] ]
   while $dna =~ m{ atg \K  [acgt]+?  (?= taa | tag | tga) }xmspg;
 ;;
 printf qq{%d sub-sequence(s) \n}, scalar @sub_seqs;
 ;;
 print $dna if @sub_seqs;
 for my $ar_sub_seq (@sub_seqs) {
   my $cursor =
     ('-' x $ar_sub_seq->[1]) . ('^' x length $ar_sub_seq->[0]);
   print $cursor;
   }
 ;;
 my @ss = $dna =~ m{ atg \K  [acgt]+?  (?= taa | tag | tga) }xmspg;
 printf qq{'$_'  } for @ss;
"
2 sub-sequence(s)
atctcggataatgggataaaaatataggctataaatggcgccccggctaattttt
-------------^^^
-------------------------------------^^^^^^^^^^
'gga'  'gcgccccggc'