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

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

##</code><code>##

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

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

##</code><code>##

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

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

##</code><code>##

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

    if ($dna =~ /atg(.+?)(?:taa|tag|tga)/) {
        say $1;
    }
'
gga

##</code><code>##

/atg (.+?) (?>taa|tag|tga)/msx