[localhost:~/Perl] lynda% more arrays4.pl
#!/usr/bin/perl -w
#

@aa_names = qw (alanine arginine aspartate asparagine cysteine glutamate
glutamine glycine histidine isoleucine leucine lysine methionine
phenylalanine proline serine threonine tyrosine tryptophan valine);

@aa_1codes = qw (A R D N C E Q G H I L K M F P S T Y W V);

@aa_3codes = qw (Ala Arg Asp Asn Cys Glu Gln Gly His Ile Leu Lys Met
Phe Pro Ser Thr Tyr Trp Val);

$again = "y";
until ($again ne "y") {

print "\nEnter a single letter amino acid code: ";
chomp ($code = <STDIN>);
$code = uc ($code);

$i = 0;
while (defined ($aa_1codes[$i]) and $code ne $aa_1codes[$i]) {
 $i ++;
  }

if (defined ($aa_1codes[$i])) {
print $aa_names[$i], "  ", $aa_3codes[$i], "  ", $aa_1codes[$i], "\n\n";
                              }
else {
print "$code is not a legal code. \n";
}

print "Again (y/n)? ";
chomp ($again = <STDIN>);
$again = lc ($again);
$again = substr ($again,0,1);
}

exit;
[localhost:~/Perl] lynda% perl arrays4.pl

Enter a single letter amino acid code: M
methionine  Met  M

Again (y/n)? y

Enter a single letter amino acid code: a
alanine  Ala  A

Again (y/n)? y

Enter a single letter amino acid code: b
b is not a legal code.
Again (y/n)? n
[localhost:~/Perl] lynda%