$ perl -Mstrict -Mwarnings -E '
    my $dna = q{atctcggataatgggataaaaatataggctataaatggcgccccggctaattttt};

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