A perl script for translating DNA to Protein

# a perl program to translate the DNA sequence to Protein sequence
#!/usr/bin/perl
print"________________________________________________________________________________\n\n";
print"\n\n\t ***************************DNA to Protein *********************************\n\n";
print" Script support : M.Shankar\n\n";
print"__________________________________________________________________________________\n\n";
print"This Script converts DNA to Protein\n\n";
print "Please choose one of the following for entring the DNA sequence\n";
print "Press 1 for manual entry or 2 for file\n";
   $entry=;
   chomp($entry);
   if($entry==1){
     print "enter the sequence:-\n";
   $dna=;
   chomp($dna);
    }elsif($entry==2){
   print "enter the filename(if it is in another folder please specify the path)\n";
   $dnafile=;
  open(FILE,$dnafile);
  @filecontents=;
  close FILE;
  chomp(@filecontents);
  $dna=join('',@filecontents);
  }else{
  print "please press only 1 or 2\n";
  exit;
  }
 #my $dna="TCATTCTCATTC";
 my $protein='';
 my $codon3;
 for(my $i=0; $i<(length($dna)-2); $i+=3){
 $codon3=substr($dna,$i,3);

  $protein.=  codon2aa($codon3);
   }
  print "\nThe protein is:-\n$protein\n";
  $len = length($protein);
  print "The length of the protein is:-\n$len\n";
   exit;
sub codon2aa{
my($codon)=@_;
$codon= uc $codon;
my(%genetic_code) = (
 'TCA'=>'S', #Serine
 'TCC'=>'S', #Serine
 'TCG'=>'S',  #Serine
 'TCT'=>'S', #Serine
 'TTC'=>'F', #Phenylalanine
 'TTT'=>'F', #Phenylalanine
 'TTA'=>'L', #Leucine
 'TTG'=>'L', #Leucine
 'TAC'=>'Y', #Tyrosine
 'TAT'=>'Y', #Tyrosine
 'TAA'=>'_', #Stop
 'TAG'=>'_', #Stop
 'TGC'=>'C', #Cysteine
 'TGT'=>'C', #Cysteine
 'TGA'=>'_', #Stop
 'TGG'=>'W', #Tryptophan
 'CTA'=>'L', #Leucine
 'CTC'=>'L', #Leucine
 'CTG'=>'L', #Leucine
 'CTT'=>'L', #Leucine
 'CCA'=>'P', #Proline
 'CAT'=>'H', #Histidine
 'CAA'=>'Q', #Glutamine
 'CAG'=>'Q', #Glutamine
 'CGA'=>'R', #Arginine
 'CGC'=>'R', #Arginine
 'CGG'=>'R', #Arginine
 'CGT'=>'R', #Arginine
 'ATA'=>'T', #Isoleucine
 'ATC'=>'T', #Isoleucine
 'ATT'=>'T', #Isoleucine
 'ATG'=>'M', #Methionine
 'ACA'=>'T', #Threonine
 'ACC'=>'T', #Threonine
 'ACG'=>'T', #Threonine
 'ACT'=>'T', #Threonine
 'AAC'=>'N', #Asparagine
 'AAT'=>'N', #Asparagine
 'AAA'=>'K', #Lysine
 'AAG'=>'K', #Lysine
 'AGC'=>'S', #Serine#Valine
 'AGT'=>'S', #Serine
 'AGA'=>'R', #Arginine
 'AGG'=>'R', #Arginine
 'CCC'=>'P', #Proline
 'CCG'=>'P', #Proline
 'CCT'=>'P', #Proline
 'CAC'=>'H', #Histidine
 'GTA'=>'V', #Valine
 'GTC'=>'V', #Valine
 'GTG'=>'V', #Valine
 'GTT'=>'V', #Valine
 'GCA'=>'A', #Alanine
 'GCC'=>'A', #Alanine
 'GCG'=>'A', #Alanine
 'GCT'=>'A', #Alanine
 'GAC'=>'D', #Aspartic Acid
 'GAT'=>'D', #Aspartic Acid
 'GAA'=>'E', #Glutamic Acid
 'GAG'=>'E', #Glutamic Acid
 'GGA'=>'G', #Glycine
 'GGC'=>'G', #Glycine
 'GGG'=>'G', #Glycine
 'GGT'=>'G', #Glycine
 );
 if(exists $genetic_code{$codon}){
 return $genetic_code{$codon};
 }
else{
  print STDERR "Bad Codon\"$codon\"!!\n";
exit;
  }
   }