sub proteintrans {
    my ($dna) = @_;

    my $protein = '';
    # split amino acids into 3 with for loop and concatenate
    for (my $i = 0; $i < (length($dna) - 2) ; $i +=3) {
        $protein .= codons(substr($dna, $i, 3));
    }

    if ($protein =~ m/_/) {
        # if string $protein matches stop codon '_'
        # find match with index()
        # form a substring before stop codon with substr()
        # return new substring protein_
        $index = index($protein, '_');
        $protein_ = substr($protein, 0, $index);
        return $protein_;
    }
    else {
        # else return protein unmodified
        return $protein;
    }
}