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

%aa_codes = ("A" => "alanine", "R" => "arginine", "D" => "aspartate",
"N" => "asparagine", "C" => "cysteine", "E" => "glutamate",
"Q" => "glutamine", "G" => "glycine", "H" => "histidine",
"I" => "isoleucine", "L" => "leucine", "K" => "lysine", "M" =>
"methionine",
"F" => "phenylalanine", "P" => "proline", "S" => "serine",
"T" => "threonine", "Y" => "tyrosine", "W" => "tryptophan", "V" =>
"valine");

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

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

if (defined ($aa_codes{$code})) {
print $aa_codes{$code}, "  ", $code, "\n\n";
                              }
else {
print "$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 hash.pl

Enter a single letter amino acid code: a
alanine  A

Again (y/n)? y

Enter a single letter amino acid code: m
methionine  M

Again (y/n)? y

Enter a single letter amino acid code: b
Again (y/n)? n
[localhost:~/Perl] lynda%