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;
    }
}

##</code><code>##

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

    # split amino acids into 3 with for loop and concatenate
    my $protein = '';
    for(my $i = 0; $i < (length($dna) - 2) ; $i +=3) {
        ++$cnt;
        my $codon = codons(substr($dna, $i, 3));
        # Return our protein if we hit our end marker
        return $protein if $codon eq '_';
        $protein .= $codon;
    }
    # else return protein (rest of $dna)
    return $protein;
}

##</code><code>##

#!/usr/bin/perl -w
use strict;
use warnings;

# DNA to scan...
my $s1 = 'AGCCATGTAGCTAACTCAGGTTACATGGGGATGACCCCGCGACTTGGATTAGAGTCTCTTTTGGAATAAGCCTGAATGATCCGAGTAGCATCTCAG';

my %genetic_code = (
  'GCA'=>'A', 'GCC'=>'A', 'GCG'=>'A', 'GCT'=>'A',# Alanine
  'AGA'=>'R', 'AGG'=>'R', 'CGA'=>'R', 'CGC'=>'R',# Arginine
  'CGG'=>'R', 'CGT'=>'R',                        # Arginine
  'AAC'=>'N', 'AAT'=>'N',                        # Asparagine
  'GAC'=>'D', 'GAT'=>'D',                        # Aspartic Acid
  'TGC'=>'C', 'TGT'=>'C',                        # Cysteine
  'CAA'=>'Q', 'CAG'=>'Q', 'GAA'=>'E', 'GAG'=>'E',# Glutamate
  'GGA'=>'G', 'GGC'=>'G', 'GGG'=>'G', 'GGT'=>'G',# Glycine
  'CAC'=>'H', 'CAT'=>'H',                        # Histidine
  'ATA'=>'I', 'ATC'=>'I', 'ATT'=>'I',            # Isoleucine
  'CTA'=>'L', 'CTC'=>'L', 'CTG'=>'L', 'CTT'=>'L',# Leucine
  'TTA'=>'L', 'TTG'=>'L',                        # Leucine
  'AAA'=>'K', 'AAG'=>'K',                        # Lysine
  'ATG'=>'M',                                    # Methionine
  'CCA'=>'P', 'CCC'=>'P', 'CCG'=>'P', 'CCT'=>'P',# Phynyalanine
  'TTC'=>'F', 'TTT'=>'F',                        # Phynyalanine
  'AGC'=>'S', 'AGT'=>'S', 'TCA'=>'S', 'TCC'=>'S',# Serine
  'TCG'=>'S', 'TCT'=>'S',                        # Serine
  'ACA'=>'T', 'ACC'=>'T', 'ACG'=>'T', 'ACT'=>'T',# Serine
  'TGG'=>'W',                                    # Tryptophan
  'TAC'=>'Y', 'TAT'=>'Y',                        # Tyrosine
  'GTA'=>'V', 'GTC'=>'V', 'GTG'=>'V', 'GTT'=>'V',# Valine
  'TAA'=>'_', 'TAG'=>'_', 'TGA'=>'_',            # exit
);

print "DNA: $s1\n";
my $start = 'M';
my $end   = '_';

my $idx = -1;
while (my $prefix = substr($s1, ++$idx, 3)) {
    last if length $prefix < 3;

    # Check for the start indicator.
    next unless codons($prefix) eq 'M';

    my $peptide = proteintrans($end, substr($s1, $idx));
    next if !defined $peptide;
    print "\n\nIDX $idx: $peptide\n";
    dbg_dump($s1, $idx, $peptide);
}

sub dbg_dump {
    my ($dna, $idx, $peptide) = @_;
    print $dna, "\n", " " x ($idx-1), '<';
    print join("  ", split //, $peptide), ">\n";
}

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

    my $protein = '';
    for (my $i; $i<length($dna)-2; $i+=3) {
        my $codon = codons(substr($dna, $i, 3));
        return $protein . "? undefined codon '$triplet'" if ! defined $codon;
        return $protein if $codon eq $end;
        $protein .= $codon;
    }
    return $codon;
}

sub codons {
    my $codon = uc shift;
    return exists $genetic_code{$codon}
                ? $genetic_code{$codon}
                : undef;
}